users.conf: Remove deprecated users.conf integration.

users.conf: Remove deprecated users.conf integration.

users.conf was deprecated in Asterisk 21 and is now being removed
for Asterisk 23, in accordance with the Asterisk deprecation policy.

This consists of:
* Removing integration with app_directory, app_voicemail, chan_dahdi,
  chan_iax2, and AMI.
* users.conf was also partially used for res_phoneprov, and this remaining
  functionality is consolidated to a separate phoneprov_users.conf,
  used only by res_phoneprov.

Resolves: #1292

UpgradeNote: users.conf has been removed and all channel drivers must
be configured using their specific configuration files. The functionality
previously in users.conf for res_phoneprov is now in phoneprov_users.conf.