astobj2: Remove OBJ_CONTINUE support.

astobj2: Remove OBJ_CONTINUE support.

OBJ_CONTINUE was a strange feature that came into the world under
suspicious circumstances to support an abuse of the ao2_container by
chan_iax2.  Since chan_iax2 no longer uses OBJ_CONTINUE, it is safe to
remove it.

The simplified code should help performance slightly and make
understanding the code easier.

Review: https://reviewboard.asterisk.org/r/2887/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@399937 65c4cc65-6c06-0410-ace0-fbb531ad65f3