dkio: remove kernel dkio.h compatibility header

Without DKIOCFLUSHWRITECACHE, we no longer need the compat header. Note
that we're keeping the userspace SPL compat header, which is used by
libefi.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16064

include/sys/vdev_impl.h

@@ -35,7 +35,6 @@
 #include <sys/nvpair.h>
 #include <sys/space_map.h>
 #include <sys/vdev.h>
-#include <sys/dkio.h>
 #include <sys/uberblock_impl.h>
 #include <sys/vdev_indirect_mapping.h>
 #include <sys/vdev_indirect_births.h>