STMClient::process_network_input: Return void, instead of always true

This function has never in its history had a codepath returning false.

src/frontend/stmclient.h

@@ -68,7 +68,7 @@ private:
   bool clean_shutdown;
 
   void main_init( void );
-  bool process_network_input( void );
+  void process_network_input( void );
   bool process_user_input( int fd );
   bool process_resize( void );