Remove merge request pipeline check in mergeable method

What does this MR do?

We released the merge blocker in the previous iteration. It was meant for an intermediate solution until we release Merge Train. Since the issue is scheduled at this milestone, the functionality should be removed now.

Read more in https://gitlab.com/gitlab-org/gitlab-ee/issues/9186#note_167093786

This reverts https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9643

This MR has a CE backport

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29074

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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