res_fax.c, res_fax_spandsp.c: Remove redundant locking.

When FAX was developed, apparently the faxregistry.container used to be a
linked list that was converted to an ao2 container.  Some of the
replacement ao2 container operations still had explicit lock/unlocks
around them.

Three off nominal code paths in res_fax.c and res_fax_spandsp.c unlock the
channel even though the routine did not lock the channel and other code
paths in the routine do not unlock the channel.

Review: https://reviewboard.asterisk.org/r/4340/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@430687 65c4cc65-6c06-0410-ace0-fbb531ad65f3