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