fix(menu): open animation not working on repeat opens (#20382)

This seems to be a remnant of the old menu animation. Previously when a menu was opened, we had a two step animation: scale and fade in the panel and then fade in the items a little bit later.

When the latest Material design spec update was implemented, the animation was simplified to a fade in and scale only, however some of the metadata doesn't seem to have been cleaned up which is causing an issue where we set an `opacity` on the items when we close the panel for the first time, but there's nothing resetting it the next time around which ends up causing the entire animation to be skipped.

Fixes #20379.

(cherry picked from commit ea628c9)

Author: crisbeto

src/material/menu/menu-animations.ts

@@ -12,8 +12,6 @@ import{
   style,
   animate,
   transition,
-  query,
-  group,
   AnimationTriggerMetadata,
 } from '@angular/animations';
 
@@ -40,12 +38,10 @@ export const matMenuAnimations: {
       opacity: 0,
       transform: 'scale(0.8)'
     })),
-    transition('void => enter', group([
-      query('.mat-menu-content, .mat-mdc-menu-content', animate('100ms linear', style({
-        opacity: 1
-      }))),
-      animate('120ms cubic-bezier(0, 0, 0.2, 1)', style({transform: 'scale(1)'})),
-    ])),
+    transition('void => enter', animate('120ms cubic-bezier(0, 0, 0.2, 1)', style({
+      opacity: 1,
+      transform: 'scale(1)'
+    }))),
     transition('* => void', animate('100ms 25ms linear', style({opacity: 0})))
   ]),