Fix Incrementing Sequence Number For Retransmitted DTMF End Packets

In Asterisk 1.4+, a fix was put in place to increment the sequence number for
retransmitted DTMF end packets.  With the introduction of the RTP engine API in
1.8, the sequence number was no longer being incremented.  This patch fixes this
regression as well as cleans up a few lines that were not doing anything.

(closes issue ASTERISK-20295)
Reported by: Nitesh Bansal
Tested by: Michael L. Young
Patches: 
01_rtp_event_seq_num.patch uploaded by Nitesh Bansal (license 6418)
asterisk-20295-dtmf-fix-cleanup.diff uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/2083/
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Merged revisions 372185 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372198 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 372199 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372200 65c4cc65-6c06-0410-ace0-fbb531ad65f3