Drop `merge_trains_enabled` column from `project_ci_cd_settings` table

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 2 of this exercise - drop the column.

Improvements

Remove unneeded column

Risks

A small amount of data exists in the column, making a rollback difficult.

Involved components

project_ci_cd_settings.merge_trains_enabled