[tex-eplain] removing diagram and bibtex code

[Karl Berry]

Dan's reply is on the mark.  Just a couple more historical notes:

The BibTeX stuff also becomes a standalone file, btxmac.tex, since
that's what Oren wanted.  That is why it includes various "generic"
definitions that are then also used in the rest of Eplain.  Sure, it
could be possible to split it up further, but I saw no reason to do so.

The commutative diagrams are indeed rather a different animal, but They
don't hurt anything that I can see.  They are there simply because Steve
Smith wrote them and offered them to Eplain (decades ago), and I was
enthusiastic about user contributions.

Best,
Karl