Cleanup invite_members_version_a experiment [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

• Removes the feature flag/experiment from invite member link in assignee dropdown for owners and maintainers

To reproduce locally(gdk)

• Navigate to any issue/merge request where you are an owner/maintainer and click the assignee dropdown as seen below
• Validate invite members link exists.

Screenshots (strongly suggested)

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 #321185