Staging: rt3070: remove dead code

While merging rt30[79]0 support into rt28[67]0, drivers/staging/rt3070/md4.h
has been forgotten, while it isn't referenced from the remaining drivers
anymore; remove it.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Matti Lammi <mattij.lammi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>