[cleanup] Remove flat storage inline migration handler #6818

Job	Run time
Backward Compatibility	11m 4s
Database Migration	11m 33s
Rust Doctests	11m 14s
Cargo Nextest (Linux)	15m 29s
Cargo Clippy	8m 45s
Cargo Fmt	22s
Cargo Nextest (Linux Nightly)	15m 47s
Cargo Nextest (MacOS)	11m 40s
Lychee Lints	8s
Protobuf Backward Compatibility	9s
Protocol Schema	12m 49s
Genesis Changes	10m 3s
Sanity Checks	6m 1s
Style	35s
Cargo Audit	20s
Cargo Deny	27s
Non-default Configuration Builds	1m 44s
Themis	39s
Unused Dependencies	6m 55s
Upgradability	12m 54s
Windows check for building public libraries	4m 58s
Upload Coverage	22s
Generate Coverage Artifact (unit, unit)	2m 0s
Generate Coverage Artifact (integration, unit integration)	2m 30s
	2h 28m 28s

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