repo-updater: Remove deprecated migrations

Created by: tsenart

This commit removes all repo-updater data migrations introduced up until the 3.4 release to support the transition to the new syncer.

As per our upgrade policy, these are now safe to remove. Customers who are more than two versions behind will have to go through intermediate version upgrades.

Removing these migrations speeds up repo-updater's startup time.

Part of #4598

Created by: tsenart

@keegancsmith: Your call if this is worth cherry-picking into the release branch.

Created by: codecov[bot]

Codecov Report

Merging #4886 into master will decrease coverage by 0.08%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4886      +/-   ##
==========================================
- Coverage   48.16%   48.07%   -0.09%     
==========================================
  Files         733      732       -1     
  Lines       44814    44641     -173     
  Branches     1763     1763              
==========================================
- Hits        21586    21463     -123     
+ Misses      21232    21211      -21     
+ Partials     1996     1967      -29

Impacted Files	Coverage Δ
cmd/repo-updater/repos/testing.go	80.98% <0%> (-1.53%)
cmd/repo-updater/repos/types.go	77.6% <0%> (-0.91%)
cmd/frontend/graphqlbackend/graphqlbackend.go	46.85% <0%> (ø)
cmd/frontend/graphqlbackend/repository.go	23.03% <0%> (+0.98%)
...d/frontend/internal/authz/bitbucketserver/store.go	80.83% <0%> (+1.19%)

Created by: keegancsmith

@keegancsmith: Your call if this is worth cherry-picking into the release branch.

I think we should hold off until the next release. Low risk change, so we can merge in if it gives conflicts.