Remove db/migrate/20170828135939_migrate_user_external_mail_data.rb
Added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12968, this is a duplicate of the post-deploy version added in the same MR and appears to have been left in by mistake.
Luckily we caught it just before tagging RC1, and this commit has already been picked into 10-0-stables.