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

Modules/FindOpenMP.cmake

@@ -126,7 +126,7 @@ set(OpenMP_C_CXX_TEST_SOURCE
 #include <omp.h>
 int main() {
 #ifdef _OPENMP
-  int n = omp_get_max_threads();
+  omp_get_max_threads();
   return 0;
 #else
   breaks_on_purpose