LU-16008 tests: don't enforce umount in recovery-small/150

LU-16008 tests: don't enforce umount in recovery-small/150

as such an enforcement disconnects all MDS clients, then
another MDS trying to talk to that original MDS gets evicted
and an unlucky RPC (e.g. rmdir in test cleanup) can fail with:
rm: cannot remove '...d110h.recovery-small/source_dir': Is a directory

Fixes: 57f3262baa7 ("LU-15788 lmv: try another MDT if statfs failed")

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I593e1425b44fc19cb7b2b7da33fa10590532f930
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/47940
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>