Bug 1746951 - Remove no-longer-needed workaround for native menus in dark mode. r=mstange,mac-reviewers

All this was introduced to make native context menus dark if necessary when
widget.macos.respect-system-appearance was false by default in bug 1704102, but
we have since enabled it and removed the preference so this is no longer
needed.

Differential Revision: https://phabricator.services.mozilla.com/D134333

modules/libpref/init/StaticPrefList.yaml

@@ -12315,11 +12315,6 @@
   type: RelaxedAtomicBool
   value: true
   mirror: always
-
-- name: widget.macos.enable-pre-bigsur-workaround-for-dark-mode-context-menus
-  type: RelaxedAtomicBool
-  value: true
-  mirror: always
 #endif
 
 # Whether to allow gtk dark themes in content.

widget/cocoa/MOZMenuOpeningCoordinator.mm

@@ -164,44 +164,8 @@ static BOOL sNeedToUnwindForMenuClosing = NO;
   // a later time.
   // The code below uses option 4 as the preferred option because it's the simplest: It works in all
   // scenarios and it doesn't have the positioning drawbacks of option 5.
-
   if (aView) {
     NSWindow* window = aView.window;
-
-    if (@available(macOS 10.14, *)) {
-      if (window.effectiveAppearance != NSApp.effectiveAppearance) {
-        // By default, NSMenu inherits its appearance from the opening NSEvent's window. But we
-        // would like it to use the system appearance - if the system uses Dark Mode, we would like
-        // context menus to be dark even if the window's appearance is Light.
-#if !defined(MAC_OS_VERSION_11_0) || MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_VERSION_11_0
-        if (nsCocoaFeatures::OnBigSurOrLater()) {
-#else
-        if (@available(macOS 11.0, *)) {
-#endif
-          // On macOS Big Sur, we can achieve this by using -[NSMenu setAppearance:].
-          [aMenu setAppearance:NSApp.effectiveAppearance];
-        } else if (mozilla::StaticPrefs::
-                       widget_macos_enable_pre_bigsur_workaround_for_dark_mode_context_menus()) {
-          // On 10.14 and 10.15, there is no API to override the NSMenu appearance.
-          // We use the following hack: We change the NSWindow appearance just long enough that
-          // NSMenu opening picks it up, and then reset the NSWindow appearance to its old value.
-          // Resetting it in the menu delegate's menuWillOpen method seems to achieve this without
-          // any flashing effect.
-          if ([aMenu.delegate isKindOfClass:[MenuDelegate class]]) {
-            MenuDelegate* delegate = (MenuDelegate*)aMenu.delegate;
-
-            // Store the old NSWindow appearance, override it with the system appearance, and then
-            // reset it when the menu is open.
-            NSAppearance* oldAppearance = window.appearance;
-            window.appearance = NSApp.effectiveAppearance;
-            [(MenuDelegate*)delegate runBlockWhenOpen:^() {
-              window.appearance = oldAppearance;
-            }];
-          }
-        }
-      }
-    }
-
     // Create a synthetic event at the right location and open the menu [option 4].
     NSPoint locationInWindow = nsCocoaUtils::ConvertPointFromScreen(window, aPosition);
     NSEvent* event = [NSEvent mouseEventWithType:NSEventTypeRightMouseDown