Don't build and install a mem.ko module.

Currently one cannot load the mem.ko module without panicing if mem is
compiled into the kernel and one cannot build a kernel w/o "device mem"
right now either.  Thus it is too dangerous to install mem.ko right now
because if one puts 'mem_load="YES"' in /etc/loader.conf they cannot
boot an "old" kernel (at the time that a kernel doesn't have to be built
with "device mem).

sys/modules/Makefile

@@ -132,7 +132,6 @@ SUBDIR=	${_3dfx} \
 	mac_test \
 	mcd \
 	md \
-	mem \
 	mii \
 	mlx \
 	${_mly} \