Remove unused rr_code

The `rr_code` field in `raidz_row_t` is unused.

This commit removes the field, as well as the code that's used to set
it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11736

include/sys/vdev_raidz_impl.h

@@ -127,7 +127,6 @@ typedef struct raidz_row {
 	abd_t *rr_abd_copy;		/* rm_asize-buffer of copied data */
 	abd_t *rr_abd_empty;		/* dRAID empty sector buffer */
 	int rr_nempty;			/* empty sectors included in parity */
-	int rr_code;			/* reconstruction code (unused) */
 #ifdef ZFS_DEBUG
 	uint64_t rr_offset;		/* Logical offset for *_io_verify() */
 	uint64_t rr_size;		/* Physical size for *_io_verify() */