Remove dead code from multicast.h

https://github.com/Hamlib/Hamlib/issues/695

include/hamlib/multicast.h

@@ -19,14 +19,6 @@
 
 #ifndef MULTICAST_H
 #define MULTICAST_H
-#if 0 // moved to rig.h
-struct multicast_s
-{
-    int multicast_running;
-    int sock;
-    int seqnumber;
-    int runflag; // = 0;
-    pthread_t threadid;
 #ifdef HAVE_ARPA_INET_H
 #warn HAVE_ARPA_INET
     struct ip_mreq mreq; // = {0};