drop support for win32/osx and checkupdate plugin

that plugin has never been built in linux anyway due to logic
in configure.ac

plugins/filebrowser/pluma-file-browser-store.c

@@ -2257,7 +2257,6 @@ model_iterate_next_files_cb (GFileEnumerator * enumerator,
  * FIXME: This is temporarly, it is a bug in gio:
  * http://bugzilla.gnome.org/show_bug.cgi?id=565924
  */
-#ifndef G_OS_WIN32
 			if (g_file_is_native (parent->file) && dir->monitor == NULL) {
 				dir->monitor = g_file_monitor_directory (parent->file, 
 									 G_FILE_MONITOR_NONE,
@@ -2271,7 +2270,6 @@ model_iterate_next_files_cb (GFileEnumerator * enumerator,
 							  parent);
 				}
 			}
-#endif
 
 			model_check_dummy (dir->model, parent);
 			model_end_loading (dir->model, parent);