FindOpenMP: Avoid warning in check code

The test code added by commit v3.12.0-rc1~411^2~1 (FindOpenMP: Verify in
test source that OMP library is linked, 2018-03-01) leaves an unused
variable warning.  This breaks the check with `-Werror`.  Remove the
variable and leave just the function call, which should still check that
the OMP library is linked.

Fixes: #18102
15 jobs for FindOpenMP-avoid-warning-in-check
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