Fix cleaning up metasets

I believe it's a mistake to only delete the metaset if it has a valid controller; see f191f2f for where this behavior was introduced.

This is a minimal fix for #9653; as discussed there, it may also be worth updating `updateHoverStyle`.

As of #7030, `this._metasets` should always be defined, so checking whether it's undefined is no longer necessary.

src/core/core.controller.js

@@ -800,12 +800,11 @@ class Chart {
 	 * @private
 	 */
   _destroyDatasetMeta(datasetIndex) {
-    const meta = this._metasets && this._metasets[datasetIndex];
-
+    const meta = this._metasets[datasetIndex];
     if (meta && meta.controller) {
       meta.controller._destroy();
-      delete this._metasets[datasetIndex];
     }
+    delete this._metasets[datasetIndex];
   }
 
   _stop() {