Remove an extraneous unlocking operation from ast_channel_free.

Remove an extraneous unlocking operation from ast_channel_free.

In the case that we could not remove the desired channel from the
list of channels, there was an extra call to unlock the channel list.
Since we unlock the list later on in the function anyway, this results
in the list being unlocked twice yet only being locked once.

(closes issue #15098)
Reported by: tim_ringenbach
Patches:
      remove_extra_unlock.diff uploaded by tim (license 540)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@194356 65c4cc65-6c06-0410-ace0-fbb531ad65f3