[Shadows] Remove __attribute__((fallthrough))

PiperOrigin-RevId: 368220450

Author: yarneo

components/Shadow/src/MDCShadowsCollection.m

@@ -106,7 +106,7 @@ - (MDCShadowsCollection *)build {
     return [UIColor colorWithDynamicProvider:^(UITraitCollection *traitCollection) {
       switch (traitCollection.userInterfaceStyle) {
         case UIUserInterfaceStyleUnspecified:
-          __attribute__((fallthrough));
+          /* FALLTHROUGH - TODO(b/185199658): Migrate to proper fallthrough logic */
         case UIUserInterfaceStyleLight:
           return LightStyleShadowColor();
         case UIUserInterfaceStyleDark: