Remove mark on project_ci_cd_settings.merge_trains_enabled

Summary

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 project_ci_cd_settings table.

This is part 3 of this exercise - remove the mark.

Improvements

Removing ignore rule on already dropped column.

Risks

None

Involved components

project_ci_cd_settings

Replaces this MR !19964 (closed)