Tidy up merge request code

What does this MR do?

This MR tidies up some merge request code. In a previous MR that removed unused code, some merge request code was identified as needing tidying up.

This MR:

• Moves _close_reopen_draft_report_toggle.html.haml to a merge_requests folder since it's only used within MRs, and tidies it up by replacing "issuable" with "merge_request" among other things. It also removes some CSS so 1) we have less CSS in our bundle and 2) the dropdown width no longer has a custom width and is more in line with other dropdown widths
• Moves some specs between /merge_request/ and /merge_requests/ to align with whether they visit project_merge_request_path or project_merge_requests_path.
• Deletes some unused code

Screenshots (strongly suggested)

before	after

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Related to #276444 (closed)