Improve migrations

[turbolent]

• Remove unused AccountsAtreeLedger. It's from '21 and not used either by atree register inlining nor Cadence 1.0 migration
• Improve comment
• Generate set of all storage map domains from slice (ensure they are in sync)

@fxamacker @janezpodhostnik seeing AccountUsageMigrator made me wonder: I guess we also want to run this after the Cadence 1.0 migration, because it also changes account storage usage (even if just slightly)?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.64%. Comparing base (8afb152) to head (8a0839b).
Report is 970 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5657      +/-   ##
==========================================
+ Coverage   56.00%   57.64%   +1.63%     
==========================================
  Files        1024      834     -190     
  Lines       99327    78547   -20780     
==========================================
- Hits        55632    45281   -10351     
+ Misses      39455    29964    -9491     
+ Partials     4240     3302     -938     

Flag	Coverage Δ
unittests	57.64% <ø> (+1.63%)	⬆️

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[janezpodhostnik]

Ideally we would run AccountUsageMigrator last, so that we don't have to worry about updating storage used in the other migrations.