Enable deleting_account_disabled_for_users by default

What does this MR do and why?

We introduced the FF in !117832 (merged) and enabled it globally on May 30th.

This MR sets the FF enabled by default.

How to set up and validate locally

1. Sign in as a user A.
2. Go to the Preferences -> Account, click Delete account.
3. You can delete the account A.
4. Open another browser window and sign in with admin account.
5. Go to Admin -> General -> Account and limit section.
6. Uncheck the checkbox Allows users to delete their own accounts and click Save changes.
7. Go back to the browser window with user B.
8. Go to the Preferences -> Account, click Delete account.
9. You cannot find the Delete account section.

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 #412411 (closed)