fix(menu): trigger not unsubscribing from panel close stream (#15667)

Fixes the `matMenuTrigger` not unsubscribing from its `_menuCloseSubscription`. Also removes a pretty simple private method that was only being used once.

Fixes #14094.

Author: crisbeto

src/material/menu/menu-trigger.ts

@@ -201,8 +201,9 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
     this._element.nativeElement.removeEventListener('touchstart', this._handleTouchStart,
         passiveEventListenerOptions);
 
-    this._cleanUpSubscriptions();
+    this._menuCloseSubscription.unsubscribe();
     this._closingActionsSubscription.unsubscribe();
+    this._hoverSubscription.unsubscribe();
   }
 
   /** Whether the menu is open. */
@@ -477,12 +478,6 @@ export class MatMenuTrigger implements AfterContentInit, OnDestroy {
     ]);
   }
 
-  /** Cleans up the active subscriptions. */
-  private _cleanUpSubscriptions(): void {
-    this._closingActionsSubscription.unsubscribe();
-    this._hoverSubscription.unsubscribe();
-  }
-
   /** Returns a stream that emits whenever an action that should close the menu occurs. */
   private _menuClosingActions() {
     const backdrop = this._overlayRef!.backdropClick();