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/os/linux/Makefile.am

@@ -63,7 +63,6 @@ kernel_spl_sys_HEADERS = \
 	%D%/spl/sys/ctype.h \
 	%D%/spl/sys/debug.h \
 	%D%/spl/sys/disp.h \
-	%D%/spl/sys/dkio.h \
 	%D%/spl/sys/errno.h \
 	%D%/spl/sys/fcntl.h \
 	%D%/spl/sys/file.h \