FindOpenSSL: Restore support for crypto-only result

Since commit v3.8.0-rc1~132^2 (FindOpenSSL: Check that both CRYPTO and
SSL libraries are present, 2017-01-03) we require both crypto and ssl
libraries to be present.  This makes sense because `OPENSSL_LIBRARIES`
lists both and breaks when one is not found.  However, prior to that
fix we supported finding only the crypto library and using it through
the imported target.  Drop the requirement for ssl to restore support
for using crypto alone.

Later this module should be taught to support the `COMPONENTS` argument
of `find_package`.

Fixes: #16882
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