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

Differential Revision: https://phabricator.services.mozilla.com/D39711

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

devtools/client/performance-new/panel.js

@@ -45,7 +45,7 @@ class PerformancePanel {
     return this.toolbox.target;
   }
 
-  async destroy() {
+  destroy() {
     // Make sure this panel is not already destroyed.
     if (this._destroyed) {
       return;