Revert OPENSSL_NO_SSL3_METHOD to keep ABI compatibility.

This define caused a couple of symbols to disappear. To keep ABI
compatibility, we are going to keep the symbols exposed, but leave SSLv3 as
not in the default config (this is what OPENSSL_NO_SSL3 achieves). The
ramifications of this is an application can still use SSLv3 if it
specifically calls the SSLv3_method family of APIs.

Reported by: kib, others
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D25451