Remove D_NEEDGIANT from io(4).

There is no need to mark this device node to use Giant. The only
architectures that use io(4) (i386 and amd64) only change a flag in
td->td_frame, which is only accessed by curthread.

Apart from this change, I think some fishy things may happen when using
/dev/io in multithreaded applications. I haven't tested, but looking at
the code, the flag doesn't get cleared when close() is called from
another thread, but this may not be this important.

I'm not removing D_NEEDGIANT from mem(4), because this driver isn't
Giant safe at all (it calls GIANT_REQUIRED).

sys/dev/io/iodev.c

@@ -52,7 +52,6 @@ static struct cdev *iodev;
 
 static struct cdevsw io_cdevsw = {
 	.d_version =	D_VERSION,
-	.d_flags =	D_NEEDGIANT,
 	.d_open =	ioopen,
 	.d_close =	ioclose,
 	.d_name =	"io",