menu: Remove GTK menu backend

Linux and macOS now both build the pure-SDL + microui menu. Remove GTK3
detection, the ENABLE_GTK build branches, and the GTK menu sources.

Decouple gettext NLS from ENABLE_GTK so menu labels stay translatable on all
desktop builds, and update i18n to the new sources (POTFILES.in, .pot, ja.po).

src/menu_emscripten.c

@@ -70,10 +70,6 @@ void menu_init(void) {
 	return;
 }
 
-void menu_gtkmainiteration() {
-	return;
-}
-
 void menu_render_overlay(void) {
 	return;
 }