Bug 1569676 - Make accessibility panel's destroy synchronous. r=jdescottes

Differential Revision: https://phabricator.services.mozilla.com/D39708

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

devtools/client/accessibility/panel.js

@@ -274,16 +274,11 @@ AccessibilityPanel.prototype = {
     return this._toolbox.target;
   },
 
-  async destroy() {
-    if (this._destroying) {
-      await this._destroying;
+  destroy() {
+    if (this._destroyed) {
       return;
     }
-
-    let resolver;
-    this._destroying = new Promise(resolve => {
-      resolver = resolve;
-    });
+    this._destroyed = true;
 
     this.target.off("navigate", this.onTabNavigated);
     this._toolbox.off("select", this.onPanelVisibilityChange);
@@ -316,8 +311,6 @@ AccessibilityPanel.prototype = {
     this.panelWin.gTelemetry = null;
 
     this.emit("destroyed");
-
-    resolver();
   },
 };