Resolve "Remove non-functional sub-menu from "Merge Requests""

What does this MR do?

This MR removes the obsolete sub-list for "Merge Requests" item in the left-hand navigation panel. The change is behind the feature flag as part of the larger effort of refactoring the panel &5647 (closed)

Screenshots (strongly suggested)

	With flag	Without flag
Hovering	cell	cell
Clicking the item	cell	cell

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the change is behind the feature flag.
• 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 #326996 (closed)