chore: Update obsolete property name in error message

The `INVALID_CONTROLLER_ERROR` error message incorrectly referenced the
property name `messagingSystem`, which is no longer used in the next version of
the `BaseController` class (which the `ComposableController` switched to in
the PR `#6710).

The error message was updated to no mention the messenger instance variable at
all, since the check that triggered the error doesn't look for it anyway.

Author: Gudahtt

packages/composable-controller/src/ComposableController.ts

@@ -12,7 +12,7 @@ import type { Messenger } from '@metamask/messenger';
 export const controllerName = 'ComposableController';
 
 export const INVALID_CONTROLLER_ERROR =
-  'Invalid controller: controller must have a `messagingSystem` and inherit from `BaseController`.';
+  'Invalid controller: controller must inherit from `BaseController`.';
 
 /**
  * The narrowest supertype for the composable controller state object.