Bug 1567860 - Make changes view destroy codepath synchronous. r=rcaliman

The fronts are destroyed when the toolbox closes and when a front is destroyed,
all its listeners are removed. So there is no real value in trying to unregister
them. On top of that, this destroy method is called by Inspector.destroy
and doesn't wait for its completion.

Differential Revision: https://phabricator.services.mozilla.com/D39302

--HG--
extra : moz-landing-system : lando

devtools/client/inspector/changes/ChangesView.js

@@ -228,14 +228,9 @@ class ChangesView {
   /**
    * Destruction function called when the inspector is destroyed.
    */
-  async destroy() {
+  destroy() {
     this.store.dispatch(resetChanges());
 
-    // ensure we finish waiting for the front before destroying.
-    const changesFront = await this.changesFrontPromise;
-    changesFront.off("add-change", this.onAddChange);
-    changesFront.off("clear-changes", this.onClearChanges);
-
     this.document = null;
     this.inspector = null;
     this.store = null;