Emitter cleans up particles when removed

If an emitter has _remove set to true, it doesn't remove any particles that are currently visible and these remain on the stage as static sprites. This change causes the emitter to remove all its particles prior to being removed itself.
The "this.active = false" line? I agree this line isn't strictly required, but it felt like a reasonable thing to do - if an emitter is flagged as removed, it should be considered inactive. I'm happy to remove this line if you prefer.