remove redundant typedef that caused problems with some compilers

thanks to report from Piotr Kubaj against FreeBSD port:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236195

lib/clixon/clixon_plugin.h

@@ -49,8 +49,6 @@
 /*
  * Types
  */
-/* Dynamicically loadable plugin object handle. @see return value of dlopen(3) */
-typedef void *plghndl_t;
 
 /*! Registered RPC callback function 
  * @param[in]  h       Clicon handle