Revert r388529 for now

Adding the cleanup function needs some deeper thought since it
apparently doesn't exist for all variants of libsrtp.
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Merged revisions 388596 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 388597 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388598 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_srtp.c

@@ -546,7 +546,6 @@ static void res_srtp_shutdown(void)
 {
 	srtp_install_event_handler(NULL);
 	ast_rtp_engine_unregister_srtp();
-	srtp_shutdown();
 	g_initialized = 0;
 }