This removes the ability to test against OpenSSL, as well as the OpenSSL verification benchmark.

The motivation is that OpenSSL 3 is deprecating part of the API used here (see #869), and I'm not sure it's worth maintaining. We do lose the fact that this is the only test that verifies randomly-generated cases against an independent implementation. On the other hand, there are tons of existing fixed tests now that test all kinds of edge cases already.

ninja check-secp256k1

git grep ENABLE_OPENSSL_TESTS