[FF] Cleanup group_owner_placeholder_confirmation_bypass

What does this MR do and why?

The ability in Direct Transfer for group owners to skip confirmation of placeholder user assignments has been enabled globally for a few milestones.

This MR deletes the feature flag group_owner_placeholder_confirmation_bypass which was previously gating the feature:

• no longer check feature flag in order for the setting to be available/usable for a group
• updates docs

References

Clean up issue -> #569771

Epic -> &17871

Screenshots or screen recordings

Unchanged

How to set up and validate locally

1. Make sure your instance is on the premium or ultimate plan. Instructions here
2. Make sure the group you want to test has EE features https://docs.gitlab.com/development/ee_features/#simulate-a-saas-instance
   1. On the left sidebar, at the bottom, select Admin. If you’ve turned on the new navigation, in the upper-right corner, select Admin.
   2. On the left sidebar, select Overview > Groups.
   3. Identify the group you want to modify, and select Edit.
   4. Scroll to Permissions and group features. For Plan, select Ultimate.
   5. Select Save changes.
3. Simulate Gitlab.com being true by running GITLAB_SIMULATE_SAAS=1 gdk restart
4. Visit the group settings page. It has to be a top-level (root) group not subgroup. Eg. http://127.0.0.1:3000/groups/flightjs/-/edit
5. Scroll down to the 'Permissions and group features' section. You should see the checkbox under 'Placeholder user confirmation'
6. Click the checkbox and save.
7. OPTIONAL - Not recommended: The actual skipping of user confirmation happens in the direct transfer process https://docs.gitlab.com/user/group/import/ . It requires migrating another group into your chosen group following direct transfer, then assigning contributions to an enterprise user following https://docs.gitlab.com/user/import/mapping/#reassign-contributions-and-memberships.

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Related to #569771