mtw(4) remove misplaced DEBUG_FLAGS

PR:		284639
Approved by: markj,emaste
Differential Revision:	https://reviews.freebsd.org/D48895

Author: jsm222

sys/modules/usb/mtw/Makefile

@@ -5,5 +5,5 @@ S=	${SRCTOP}/sys
 KMOD=	if_mtw
 SRCS=	opt_bus.h opt_usb.h opt_wlan.h device_if.h bus_if.h usb_if.h \
 	usbdevs.h if_mtw.c
-DEBUG_FLAGS=-g
+
 .include <bsd.kmod.mk>