Citations not referenced warnings

[gliptak]

Build https://travis-ci.com/github/crypto101/book/builds/186566487 shows:

/repo/src/zreferences.rst: WARNING: Citation [nist:sp800-90a] is not referenced.
/repo/src/zreferences.rst: WARNING: Citation [bellarecanettikrawczyk:keyinghash] is not referenced.
/repo/src/zreferences.rst: WARNING: Citation [cryptoeprint:2004:199] is not referenced.

Are these be removed from https://github.com/crypto101/book/blob/master/src/Crypto101.bib ?

[multun]

these could be removed, but I figured @lvh might wanna keep these around just in case they end up being used in later additions

[lvh]

Yeah, exactly. It's almost weird these aren't getting used now, they're pretty important reference works.

[lvh]

Maybe a good compromise: comment them? They'll be easy to get back, and no warnings.