Removed usages of gma_member_cleanup flag

What does this MR do?

Initially, when a group was converted to GMA the existing members were removed from the group.

!29425 (merged) introduced the changes to retain the group members after the conversion.

We introduced the gma_member_cleanup flag in case we wished to rollback to removing the group members.

Since the flag was never enabled on production and staging ever since it was introduced, I think it's safe to remove the flag as well as the code related to removal of group members on GMA conversion.

FF updates related to #251080 (closed)

Screenshots

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