Fix gcc warning

Remove -Wunused-but-set-variable (`mopt`).

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D7119
MFC after: 1 week
X-MFC with: r302332
Reported by: Jenkins
Reviewed by: grehan (maintainer)
Sponsored by: EMC / Isilon Storage Division

usr.sbin/bhyve/usb_mouse.c

@@ -297,9 +297,6 @@ static void *
 umouse_init(struct usb_hci *hci, char *opt)
 {
 	struct umouse_softc *sc;
-	char *mopt;
-
-	mopt = opt;
 
 	sc = calloc(1, sizeof(struct umouse_softc));
 	sc->hci = hci;