FindGTest: Revert "Allow either "Debug" or "Release" configurations."

Revert commit a064b18f85 (FindGTest: Allow either "Debug" or "Release"
configurations., 2020-06-29, v3.19.0-rc1~584^2).  If both configurations
are found then `GTEST_LIBRARY` is populated in a way that breaks
`__gtest_determine_library_type`, and the imported target is not
created correctly.

Fixes: #21473
Issue: #17799
1 file changed