Removing old schema merge guard

[fulghum]

Before we added support for schema merge conflict reporting, we had a guard in place that would error out for schema changes in merges. This code prevented schema merges that Dolt couldn't handle, but it didn't provide much visibility into the exact schema conflicts. The new schema merge conflict reporting code is able to handle these same cases and provides better visibility into exactly what the schema conflicts are. We've already removed some of the checks from this guard, but there were still a few left in place. This PR completely removes the old schema merge guard code.

The code left in this guard was still protecting against a couple of cases that are now fixed and covered with tests:

• Collation changes – we weren't properly handling merging changes to a table's default collation. Fixed and tested now.
• New, non-nullable columns with no default value – new columns that are non-nullable and don't have a default value can't be auto merged since we don't know what value to safely use in the new column for any existing rows, so this now throws an error with instructions on how to fix.

[zachmu]

LGTM!

[github-actions]

@coffeegoddd DOLT

name	add_cnt	delete_cnt	update_cnt	latency
adds_only	60000	0	0	1.11
adds_updates_deletes	60000	60000	60000	5.92
deletes_only	0	60000	0	2.81
updates_only	0	0	60000	3.89

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.29	0.28
batching	batch sql	10000	1	0.48	0.67
batching	by line sql	10000	1	0.53	0.66
blob	1 blob	200000	1	2.28	2.74	2.74
blob	2 blobs	200000	1	2.16	3.77	3.77
blob	no blob	200000	1	2.31	1	1.03
col type	datetime	200000	1	1.93	1.45	1.52
col type	varchar	200000	1	1.8	1.46	1.45
config width	2 cols	200000	1	1.92	0.97	0.93
config width	32 cols	200000	1	4.42	1.14	1.71
config width	8 cols	200000	1	2.17	1.12	1.33
pk type	float	200000	1	1.89	0.99	1
pk type	int	200000	1	1.78	1.23	1.01
pk type	varchar	200000	1	3.2	0.85	0.83
row count	1.6mm	1600000	1	11.39	1.41	1.36
row count	400k	400000	1	3.23	1.17	1.19
row count	800k	800000	1	6.12	1.27	1.25
secondary index	four index	200000	1	7.33	0.97	0.8
secondary index	no secondary	200000	1	2.21	0.98	1.05
secondary index	one index	200000	1	2.66	1.2	1.17
secondary index	two index	200000	1	4.6	1	0.87
sorting	shuffled 1mm	1000000	0	19	1.02	0.97
sorting	sorted 1mm	1000000	1	20.24	0.89	0.87

[github-actions]

@coffeegoddd DOLT

name	mean	mean_mult
dolt_blame_basic	29.69	1.38
dolt_blame_commit_filter	209.51	2.98
dolt_commit_ancestors_commit_filter	0.32	0.91
dolt_commits_commit_filter	0.34	0.94
dolt_diff_log_join_from_commit	109.16	1.64
dolt_diff_log_join_to_commit	107.59	1.63
dolt_diff_table_from_commit_filter	0.69	1.23
dolt_diff_table_to_commit_filter	0.66	1.2
dolt_diffs_commit_filter	0.41	1.14
dolt_history_commit_filter	0.6	1.43
dolt_log_commit_filter	0.34	0.94