Remove some unneeded casts from remote.c

I happened to notice a few unneeded casts in remote.c.  In some cases
these are no-ops, and in others these cast away const, but in a context
where this is not needed.

I'm checking this in under the obvious rule.
Tested by rebuilding on x86-64 Fedora 24.

2016-09-08  Tom Tromey  <tom@tromey.com>

	* remote.c (remote_notif_stop_ack, remote_wait_as)
	(show_remote_cmd): Remove unneeded casts.