fix(panel) memory leak

When destroying md-tooltip the callback `onDomAdded` remained attached and was causing a memory leak.

Added 4 lines with setting all 4 callbacks to null in `MdPanelRef.prototype.destroy`.

Fixes #11133

src/components/panel/panel.js

@@ -1608,6 +1608,10 @@ MdPanelRef.prototype.destroy = function() {
   }
   this.config.scope.$destroy();
   this.config.locals = null;
+  this.config.onDomAdded = null;
+  this.config.onDomRemoved = null;
+  this.config.onRemoving = null;
+  this.config.onOpenComplete = null;
   this._interceptors = null;
 };