[cleanup] Remove flat storage inline migration handler

[shreyan-gupta]

No longer needed after the migration.

I noticed while working on some refactoring wrt store.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.46%. Comparing base (4fe8380) to head (963b4fb).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12122      +/-   ##
==========================================
- Coverage   71.52%   71.46%   -0.06%     
==========================================
  Files         819      818       -1     
  Lines      164694   164501     -193     
  Branches   164694   164501     -193     
==========================================
- Hits       117793   117568     -225     
- Misses      41762    41797      +35     
+ Partials     5139     5136       -3     

Flag	Coverage Δ
backward-compatibility	0.17% <ø> (+<0.01%)	⬆️
db-migration	0.17% <ø> (+<0.01%)	⬆️
genesis-check	1.26% <ø> (+<0.01%)	⬆️
integration-tests	38.63% <ø> (-0.01%)	⬇️
linux	71.26% <ø> (-0.06%)	⬇️
linux-nightly	71.03% <ø> (-0.08%)	⬇️
macos	54.01% <ø> (+1.05%)	⬆️
pytests	1.52% <ø> (+<0.01%)	⬆️
sanity-checks	1.33% <ø> (+<0.01%)	⬆️
unittests	65.17% <ø> (-0.08%)	⬇️
upgradability	0.21% <ø> (+<0.01%)	⬆️

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