chan_iax2: Restore codec choice behavior from media formats branch

After merging the media formats branch, chan_iax2 was discarding
codec preferences for the purpose of choosing which codec a
channel would use once a call started. This patch restores the
Asterisk 1.8-12 codec choice behaviors.

ASTERISK-23958 #close
Review: https://reviewboard.asterisk.org/r/3800/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419109 65c4cc65-6c06-0410-ace0-fbb531ad65f3