hfp_hf: Fix modifying hash table while iterating

hfp_hf: Fix modifying hash table while iterating

Calling ofono_remove_modem() while iterating the hash table is not safe
given that it can modify the table in hfp_remove().

A simple way to reproduce the problem is to pair some Bluetooth phones
and remove the Bluetooth adapter, triggering a GLib-CRITICAL assertion.

This patch just removes the update of the hash table inside
hfp_remove(). This can be safe as long as all calls to
ofono_modem_remove() are accompanied by a hash table update. The only
example of this kind right now is hfp_agent_release().