LU-15407 test: remove dummy enc key at cleanup

LU-15407 test: remove dummy enc key at cleanup

Make sure to remove the dummy encryption key from session keyring
when cleaning up encryption tests.

Lustre-change: https://review.whamcloud.com/46038
Lustre-commit: ec0b308614a2bad18a7a1fd805f36eb8ed6ea5eb

Test-Parameters: trivial
Test-Parameters: testlist=sanity-sec mdscount=2 mdtcount=4 osscount=1 ostcount=8 clientcount=2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I840490fca0a485110d077fe85254ced817fd55e3
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46173
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>