Remove use_user_group_member_roles feature flag

What does this MR do and why?

Remove use_user_group_member_roles FF (globally enabled 3 weeks ago) to make UserMemberRolesInGroupsPreloader and UserMemberRolesInProjectsPreloader use the cached data in user_group_member_roles table to determine a user's custom permissions in a given project or group.

References

[FF] `use_user_group_member_roles` -- Use `user... (#557017)

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