LU-16658 tests: disable performance-sanity test_6

LU-16658 tests: disable performance-sanity test_6

This test is likely failing due to a bug in mdsrate, which is no
longer actively developed.  It should be replaced by mdtest.

Test-Parameters: trivial testlist=performance-sanity
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I05378fb75ed30e56983f4668c03725824ad5a8ab
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50386
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sarah Liu <sarah@whamcloud.com>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>