remove SSL 2 backwards compatibility

A straightforward cleanup of part of the backwards compatibility section. (handles an easy part of issue #54) SSL 2.0 backwards compatibility was previously deprecated in TLS 1.2 (2008) and was warned to be "phased out with all due haste". RFC 6176 (2011) prohibits it entirely. It's a section of obsolete complexity still in the draft. Remove it and replace it with a simple "MUST NOT" send or accept.