Remove "index" column from dbufstat.py

Commit ca0bf58d to address arcs_mtx contention removed column "index"
from the output of kstats/dbuf.

dbufstat.py was not updated to reflect this, which causes it to crash
when run with -bx

This removes "index" from hardcoded lists of columns.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4096