Remove token from personal_access_tokens table

In 11.4, we introduced personal_access_tokens.token_digest, so we can now remove personal_access_tokens.token.

Since the data is migrated in a background migration, there should be no associated data loss.

Also, we might no longer need to fallback to returning clear-text value in PersonalAccessToken#redis_getdel. See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22916#note_117630911

Edited Nov 19, 2018 by Imre Farkas