Remove crypto abstract

The crypto design docs have not yet been finalized, this document
is not entirely accurate and was written early on for the zeromq
team just as an abstract on the crypto routine
Apparently md5 is deprecated in favor of hashlib
Merge pull request #3448 from techhat/issue3446

Apparently md5 is deprecated in favor of hashlib
Merge remote-tracking branch 'upstream/develop' into develop