address review: remove NullTaskManager.onClose no-op override

The override prevented the base class _requestResolvers cleanup from
running. Base TaskManager.onClose just clears two maps, which is cheap
and correct for the null variant too.

Author: felixweinberger

packages/core/src/shared/taskManager.ts

@@ -894,8 +894,4 @@ export class NullTaskManager extends TaskManager {
     ): Promise<{ queued: boolean; jsonrpcNotification?: JSONRPCNotification }> {
         return { queued: false, jsonrpcNotification: { ...notification, jsonrpc: '2.0' } };
     }
-
-    override onClose(): void {
-        // No-op
-    }
 }