transmit_silence_during_record replaced by transmit_silence

In asterisk.conf, transmit_silence_during_record has been removed
in favor of using only the transmit_silence option.  The
transmit_silence_during_record option remains a valid option in
asterisk.conf, but has been removed from the sample config and
noted in CHANGES.  



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@240971 65c4cc65-6c06-0410-ace0-fbb531ad65f3