Remove unneeded args from commands.

Now that the scripts detect Haiku automatically, no need to force it on
all scripts manually.
Unregister the listener from the request when deleting.

* Even if we abort the request, RequestCompleted may still be called
with the results. This leads to a crash if the listener is gone, so
unregister it.
* Use a BReference to hold the ResourceHandle, as this makes the code
more readable than using the assignment operator to create redirected
requests.