LU-16775 tests: cleanup target after sanity-sec test_31

LU-16775 tests: cleanup target after sanity-sec test_31

sanity-sec test_31 adds an LNet network tcp999, and associated
servicenode param on MDS target. This param must be cleared when
exiting the test, otherwise it can lead to incorrect client HA
behavior, trying to reach out to the fake service nodes.

Lustre-change: https://review.whamcloud.com/50766
Lustre-commit: TBD (e8b6c5a1128210a3a2bf525b5b489224932c8f88)

Test-Parameters: trivial
Test-Parameters: mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2 \
    clientselinux testlist=sanity-sec env=SHARED_KEY=true,ONLY="31 61"
Fixes: c508c94268 ("LU-16557 client: -o network needs add_conn processing")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: If3a1926855bd23e9154c9a32b7a555e934e94565
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50767
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>