Remove UIO_ZEROCOPY functions structures

The original xuio zero copy functionality has always been unused 
on Linux and FreeBSD.  Remove this disabled code to avoid any
confusion and improve readability.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11124

include/sys/dmu_impl.h

@@ -237,13 +237,6 @@ extern "C" {
 struct objset;
 struct dmu_pool;
 
-typedef struct dmu_xuio {
-	int next;
-	int cnt;
-	struct arc_buf **bufs;
-	iovec_t *iovp;
-} dmu_xuio_t;
-
 typedef struct dmu_sendstatus {
 	list_node_t dss_link;
 	int dss_outfd;