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.

Maniphest Tasks: T13382

Differential Revision: https://secure.phabricator.com/D20725