Remove X509PurposeId::NS_SSL_SERVER API, from the SSL 2.0 era

[davidben]

rust-openssl still carries some remnants of SSL 2.0. X509PurposeId::NS_SSL_SERVER corresponds to X509_PURPOSE_NS_SSL_SERVER, which mandates the keyEncipher key usage bit.

TLS from the last decade uses the digitalSignature bit, because we use the credential for signing, not encryption. keyEncipherment is used by the legacy RSA key exchange mode. As early as SSL 3.0, we had signing-based cipher suites in the form of DHE_RSA. Only SSL 2.0 was encryption-only.

As SSL 2.0 is broken. rust-openssl really shouldn't be carrying remnants of it anymore, or depending on X.509 APIs to do so.