R1:64b399d9be26

Remove "bin/accountadmin" and "scripts/user/add_user.php"

Summary:
Fixes T13382. Depends on D20724. These ancient scripts are no longer necessary since we've had a smooth web-based onboarding process for a long time.

I retained `bin/user empower` and `bin/user enable` for recovering from situations where you accidentally delete or disable all administrators. This is normally difficult, but some users are industrious.

Test Plan: Grepped for `accountadmin` and `add_user.php`, found no more hits.