qa: fix multi-fs tests in test_mds_metrics.py

qa: fix multi-fs tests in test_mds_metrics.py

* Avoids the unnecessary setup, when writing a multi-fs test.
  Avoids creating the default filesystem, deleting it and creating the required filesystems, mounting them.
  This change uses the filesystems created using 'REQUIRE_BACKUP_FILESYSTEM' for conducting tests.
* This change consequently fixes the old/deleted filesystems appearing in the `perf stats` output,
  making it stale output.
* Drops unused function parameters.

Fixes: https://tracker.ceph.com/issues/68001
Fixes: https://tracker.ceph.com/issues/68446
Signed-off-by: Jos Collin <jcollin@redhat.com>