Remove srtp_shutdown from res_srtp

The patch for ASTERISK-19253 included properly shutting down the libsrtp
library in the case of module unload.  Unfortunately, not all distributions
have the srtp_shutdown call.  As such, this patch removes calling
srtp_shutdown.
........

Merged revisions 356650 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 356651 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@356652 65c4cc65-6c06-0410-ace0-fbb531ad65f3