Remove convert_user_to_group_managed_accounts, remove_non_gma_memberships FFs and related code

What does this MR do and why?

Group managed accounts is deprecated.

Part of this cleanup: &8518

Relates to [FF removal] Remove convert_user_to_group_mana... (#414422 - closed)

The first commit, backend, had a review here: !127391 (merged)

The second commit, frontend, should be the main focus of this review.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.