compat_ioctl: remove last RAID handling code

Commit aa98aa31 ("md: move compat_ioctl handling into md.c")
already removed the COMPATIBLE_IOCTL() table entries and added
a complete implementation, but a few lines got left behind and
should also be removed here.

Cc: linux-raid@vger.kernel.org
Cc: Song Liu <song@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>