LU-10574 tests: remove useless check from sanity-dom.sh

LU-10574 tests: remove useless check from sanity-dom.sh

Tests test_sanity() and test_sanityn() are skipped if started
not from lustre/tests directory because of incorrect check
that ./sanity.sh exists.
Patch removes the check of the files which are part of
lustre/tests.

Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-2594
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Test-Parameters: testlist=sanity-dom
Change-Id: I51ad517fbf3ff653d9a11994eb280daee589a886
Reviewed-on: https://review.whamcloud.com/31074
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>