fix(overlay): detach method returns undefined

Currently the JSDoc of the `detach` method describes that the method returns a Promise if the detaching is done. This is not the case and just `undefined` is being returned because every `PortalHost` implementation inside of the CDK is synchronously detaching the view.

In the future it might be useful to change this method back to a `Promise` in favor of animations, but for now it doesn't make sense return a `Promise` because the `attach` method is also synchronous.

Fixes angular#7408

src/cdk/overlay/overlay-ref.ts

@@ -83,11 +83,8 @@ export class OverlayRef implements PortalHost {
     return attachResult;
   }
 
-  /**
-   * Detaches an overlay from a portal.
-   * @returns Resolves when the overlay has been detached.
-   */
-  detach(): Promise<any> {
+  /** Detaches an overlay from a portal. */
+  detach(): void {
     this.detachBackdrop();
 
     // When the overlay is detached, the pane element should disable pointer events.
@@ -99,12 +96,10 @@ export class OverlayRef implements PortalHost {
       this._config.scrollStrategy.disable();
     }
 
-    let detachmentResult = this._portalHost.detach();
+    this._portalHost.detach();
 
     // Only emit after everything is detached.
     this._detachments.next();
-
-    return detachmentResult;
   }
 
   /**