This patch removes XDWRITEREAD support because it never fully worked when
it was added in the initial LIO merge and it's been fully broken since 2013
from commit a2890087 ("target: Add compare_and_write_post() completion
callback fall through").

The two issues above are:

1. XDWRITEREAD support was just never completed when LIO was merged. We
never did the DISABLE WRITE check and so we never write data out.

2. Since the commit above, we never complete the command. After we do the
XOR, we return from xdreadwrite_callback and that's it. We never send a
response for the command, so the command will always time out and fail.

Since this has been fully broken for almost nine years this patch just
removes emulated support.

Link: https://lore.kernel.org/r/20220726235339.14551-1-michael.christie@oracle.comSigned-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>