Bug 1708457 - [devtools] Don't return a Promise in Inspector#_destroyMarkup. r=jdescottes.

MarkupView#destroy is synchronous, so there's no need for _destroyMarkup to return
a Promise.
The different callsites don't do anything with the returned value so there's nothing
more to cleanup.

Differential Revision: https://phabricator.services.mozilla.com/D113803

devtools/client/inspector/inspector.js

@@ -1739,20 +1739,14 @@ Inspector.prototype = {
   },
 
   _destroyMarkup: function() {
-    let destroyPromise;
-
     if (this.markup) {
-      destroyPromise = this.markup.destroy();
+      this.markup.destroy();
       this.markup = null;
-    } else {
-      destroyPromise = promise.resolve();
     }
 
     if (this._markupBox) {
       this._markupBox.style.visibility = "hidden";
     }
-
-    return destroyPromise;
   },
 
   onEyeDropperButtonClicked: function() {