tc-remove-20181218192239-migration.yml

We've found some problems with the backgroung migration:
BackfillLegacyProjectRepositories

See https://gitlab.com/gitlab-org/gitlab-ce/issues/56061

The migration was disabled to run on production:
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5906

So to overcome it's not being run anywhere, just remove it.

Whenever all problems in BackfillLegacyProjectRepositories are fixed
we'll create a new migration (with a different timestamp) to have it
run.