Remove unnecessary channel module references.

Remove unnecessary channel module references.

* Removed call to ast_module_user_hangup_all() in res_config_mysql.c since
it is effectively a noop.  No channels can attach a reference to that
module.

* Removed call to ast_module_user_hangup_all() in app_celgenuserevent.c.
The caller of unload_module() has already called it.

* Removed redundant channel module references in pbx_dundi.c.  The
registered dialplan function callback dispatchers for the read/read2/write
callbacks already reference the module before calling.

* pbx_dundi: Moved unregistering CLI commands, DUNDi switch, and dialplan
functions to the first thing the unload_module() does.  This will reduce
the chance of new channels using DUNDi services while the module is being
torn down.
........

Merged revisions 376657 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@376658 65c4cc65-6c06-0410-ace0-fbb531ad65f3