Revert a change to audio_audiohook_write_list that had no affect.

Revert a change to audio_audiohook_write_list that had no affect.

When I made this change initially, I was under the false impression that the
audiohooks structure remained on the channel after all of the hooks had been
detached.  This is not the case, ast ast_read takes care of removing the
audiohooks structure if the lists are empty.
........

Merged revisions 355622 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@355623 65c4cc65-6c06-0410-ace0-fbb531ad65f3