Remove MigrateLegacyUploads migration
As discussed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24679#note_135111389 we have a migration that is dependant on application code and should be removed in order to prevent breaking this migration in the future.
The migration was delayed and should be removed in 12.4 (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29409/diffs#dcf43da7d7bba860f8bcd730ab4ebf3171cd6caf_0_11)
Notes
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Complete this in conjunction with https://gitlab.com/gitlab-org/gitlab-ce/issues/42446
Edited by Gabe Weaver