bridge.c: Hangup attended transfer target after it has been swapped out

bridge.c: Hangup attended transfer target after it has been swapped out

After completing an attended transfer the transfer target channel (the one that
gets swapped out) was not being hung up after leaving the bridge. This resulted
in a channel possibly being left around. Added an explicit softhangup for the
channel in question after the transfer is successfully completed in order to
make sure the channel is hung up.

ASTERISK-24782 #close
Reported by: John Bigelow
Review: https://reviewboard.asterisk.org/r/4575/

This patch is a remedial cherry-pick from v13.

Change-Id: I26cc0c207acf74ade93e6567febf7b9776452058