Remove mark on project_ci_cd_settings.merge_trains_enabled
Since "Allow Merge Train" option is enabled by default from GitLab 12.1 (See https://gitlab.com/gitlab-org/gitlab-ee/issues/11222), we should drop the
merge_trains_enabled column from
This is part 3 of this exercise - remove the mark.
ignore rule on already dropped column.
Replaces this MR !19964 (closed)