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.

Lustre-change: https://review.whamcloud.com/50386
Lustre-commit: TBD (from c4b23f1975e750e8c8cbe64741b8427d708cb7a6)

Test-Parameters: trivial testlist=performance-sanity
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I05378fb75ed30e56983f4668c03725824ad5a8ab
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50426
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>