res_fax.c: Remove redundant locking.

res_fax.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.

Two off nominal code paths in res_fax.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/

Backported from v13 revision fa80d9658df8aff71a8975ab7d1fe477ea3f99b9.

Change-Id: I59a1b95a91dac8375e0db568ddb217bcd6d7d2fa