[Feature flag] Cleanup invite_members_group_modal

What does this MR do and why?

With successfully defaulting invite_members_group_modal to true in %14.8 we can now remove this feature flag.

• Removes the invite_members_group_modal feature flag
• Removes the create Rails controller action and routes for project/group members as they are no longer needed after removing the feature flag.

The replacement for this, the invite member modal, currently uses the members API(for user_ids) and invitations API(for emails) are used via invite_members_modal

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #352526 (closed)