ZyklonB: don't look for plugins in /usr/lib

It's quite unlikely that this project will ever see compiled plugins.

zyklonb.c

@@ -1019,7 +1019,6 @@ plugin_resolve_relative_filename (const char *filename)
 {
 	struct strv paths = strv_make ();
 	get_xdg_data_dirs (&paths);
-	strv_append (&paths, LIBDIR);  // Legacy, we do not compile any plugins
 	char *result = resolve_relative_filename_generic
 		(&paths, PROGRAM_NAME "/plugins/", filename);
 	strv_free (&paths);