Remove merge_trains_enabled column from project_ci_cd_settings table

What does this MR do?

Drop the project_ci_cd_settings.merge_trains_enabled column.

The "Allow Merge Train" option is enabled by default from GitLab 12.1 (See https://gitlab.com/gitlab-org/gitlab-ee/issues/11222), and in this MR we drop the merge_trains_enabled column from project_ci_cd_settings table.

See !14429 (comment 191704285)

Merge Trains can still be disabled as a feature flag via the Rails console.

project = Project.find(<project_id>)
::Feature.disable(:merge_trains_enabled, project)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Part of #12796 (closed)