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

cmd/arc_summary/arc_summary3

@@ -58,7 +58,6 @@ SECTION_PATHS = {'arc': 'arcstats',
                  'dmu': 'dmu_tx',
                  'l2arc': 'arcstats',  # L2ARC stuff lives in arcstats
                  'vdev': 'vdev_cache_stats',
-                 'xuio': 'xuio_stats',
                  'zfetch': 'zfetchstats',
                  'zil': 'zil'}