res_musiconhold: Don't restart MOH from beginning after announcement.

This reverts a problem introduced by the fix for ASTERISK_24329.
Now, when an announcement is played while waiting in a queue, music on
hold will not restart from the beginning of the sound file and will
instead pick up where it left off. However, the incorrect behavior in
ASTERISK_24329 is now present again; if an announcement X seconds
long is played when music on hold starts, music on hold will start X
seconds into the file.

ASTERISK-27774 #close
Reported by: lvl

Change-Id: I86b2885ee7063268f9b9747eddb788336ade989b