Task: cleanup deprecated flags and code

[deepthi]

We need to review which of the following can be deleted in the current milestone.

~/go/src/vitess.io/vitess$ grep -n -i -r --include=\*.go deprecated --exclude=\*rice-box.go
go/vt/mysqlctl/mysql_daemon.go:63:	// Deprecated: use MasterPosition() instead
go/vt/mysqlctl/reparent.go:95:// Deprecated: use mysqld.MasterPosition() instead
go/vt/mysqlctl/fakemysqldaemon/fakemysqldaemon.go:336:// DemoteMaster is deprecated: use mysqld.MasterPosition() instead
go/vt/vtgate/vtgate.go:60:	_                  = flag.Bool("disable_local_gateway", false, "deprecated: if specified, this process will not route any queries to local tablets in the local cell")
go/vt/vtgate/executor.go:63:	// TODO: @rafael - These two counters should be deprecated in favor of the ByTable ones. They are kept for now for backwards compatibility.
go/vt/proto/topodata/topodata.pb.go:447:	// This field is in the process of being deprecated in favor of
go/vt/proto/vtgate/vtgate.pb.go:116:	// single_db is deprecated. Use transaction_mode instead.
go/vt/proto/vtgate/vtgate.pb.go:321:	// These values are deprecated. Use session instead.
go/vt/proto/vtgate/vtgate.pb.go:483:	// not_in_transaction is deprecated.
go/vt/proto/vtgate/vtgate.pb.go:651:	// not_in_transaction is deprecated.
go/vt/proto/vtgate/vtgate.pb.go:819:	// not_in_transaction is deprecated.
go/vt/proto/vtgate/vtgate.pb.go:989:	// not_in_transaction is deprecated.
go/vt/proto/vtgate/vtgate.pb.go:1214:	// These values are deprecated. Use session instead.
go/vt/proto/vtgate/vtgate.pb.go:1791:	// These values are deprecated. Use session instead.
go/vt/proto/vtgate/vtgate.pb.go:2312:	// single_db is deprecated. Use transaction_mode instead.
go/vt/proto/vtgate/vtgate.pb.go:2409:	// atomic is deprecated. Use transaction_mode instead.
go/vt/proto/vtrpc/vtrpc.pb.go:179:// LegacyErrorCode is the enum values for Errors. This type is deprecated.
go/vt/proto/vtrpc/vtrpc.pb.go:184:// In order to avoid confusion, this type will be deprecated in
go/vt/proto/binlogdata/binlogdata.pb.go:172:	// BL_DML is deprecated.
go/vt/proto/binlogdata/binlogdata.pb.go:173:	BinlogTransaction_Statement_BL_DML_DEPRECATED BinlogTransaction_Statement_Category = 4
go/vt/proto/binlogdata/binlogdata.pb.go:186:	4: "BL_DML_DEPRECATED",
go/vt/proto/binlogdata/binlogdata.pb.go:199:	"BL_DML_DEPRECATED": 4,
go/vt/proto/query/query.pb.go:3593:// - aggregate_stats is not set (deprecated)
go/vt/dbconfigs/dbconfigs.go:135:	flag.StringVar(&dbc.param.DeprecatedDBName, "db-config-"+userKey+"-dbname", "", "deprecated: dbname does not need to be explicitly configured")
go/vt/dbconfigs/dbconfigs.go:232:	if params.connParams.DeprecatedDBName != "" {
go/vt/dbconfigs/dbconfigs.go:233:		params.connParams.DbName = params.connParams.DeprecatedDBName
go/vt/topo/k8stopo/client/informers/externalversions/factory.go:86:// Deprecated: Please use NewSharedInformerFactoryWithOptions instead
go/vt/vttablet/tabletserver/tabletenv/config.go:45:	// Placeholder for deprecated variable.
go/vt/vttablet/tabletserver/tabletenv/config.go:47:	deprecatedMessagePoolPrefillParallelism int
go/vt/vttablet/tabletserver/tabletenv/config.go:48:	deprecatedAutocommit                    bool
go/vt/vttablet/tabletserver/tabletenv/config.go:58:	flag.IntVar(&deprecatedMessagePoolPrefillParallelism, "queryserver-config-message-conn-pool-prefill-parallelism", 0, "DEPRECATED: Unused.")
go/vt/vttablet/tabletserver/tabletenv/config.go:69:	flag.BoolVar(&deprecateAllowUnsafeDMLs, "queryserver-config-allowunsafe-dmls", false, "deprecated")
go/vt/vttablet/tabletserver/tabletenv/config.go:87:	flag.BoolVar(&deprecatedAutocommit, "enable-autocommit", true, "This flag is deprecated. Autocommit is always allowed.")
go/vt/worker/defaults.go:41:	// defaultDestinationPackCount is deprecated in favor of the writeQueryMax*
go/cmd/vtbackup/vtbackup.go:107:	_ = flag.Duration("timeout", 2*time.Hour, "DEPRECATED AND UNUSED")
go/cmd/vtbackup/vtbackup.go:108:	_ = flag.Duration("replication_timeout", 1*time.Hour, "DEPRECATED AND UNUSED")
go/sqltypes/value.go:158:// This function is deprecated. Use the type-specific
go/mysql/query.go:381:		// EOF is only present here if it's not deprecated.
go/mysql/query.go:416:			// The deprecated EOF packets change means that this is either an
go/mysql/replication_constants.go:24:	// TypeDecimal is MYSQL_TYPE_DECIMAL. It is deprecated.
go/mysql/conn_params.go:40:	// The following is only set when the deprecated "dbname" flags are
go/mysql/conn_params.go:42:	DeprecatedDBName string
go/mysql/streaming_query.go:75:		// EOF is only present here if it's not deprecated.

[deepthi]

cc @morgo @sougou

[deepthi]

Some of the deprecated flags have been deleted. We'll re-evaluate the rest for 8.0.