Update arcstat.py to remove deprecated rmis reference.
Running arcstat.py -x currently throws KeyError due to rmis being
absent, it was removed in commit ca0bf58.
Signed-off-by: cable2999 <cable2999@users.noreply.github.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3931