Apple+H (hide window) shortcut does not work, so hide it

This likely broke in 3.x when Ardour migrated to context dependent
shortcuts (rather than GTK managed menu shortcuts).

While there is Gtkmm2ext::Application::hide, there is currently no
bindable action and hence no way to add a dedicated
shortcut. Furthermore, since this is a special NSApplication menu,
updating the menu-item when the action is bound is not trivial.

gtk_application_hide() is only implemented for macOS and
there is likely a better use for "H" shortcut anyway.

libs/gtkmm2ext/gtkapplication_quartz.mm

@@ -1223,7 +1223,7 @@ create_apple_menu ()
 	[menuitem release];
 	[_app_menu addItem: [NSMenuItem separatorItem]];
 	menuitem = [[NSMenuItem alloc] initWithTitle:@"Hide"
-		    action:@selector(hide:) keyEquivalent:@"h"];
+		    action:@selector(hide:) keyEquivalent:@""];
 	[menuitem setTarget: NSApp];
 	[_app_menu addItem: menuitem];
 	[menuitem release];