Suggest: remove enquire.c and related rules from the tree.

[Nathanael Nerode]

Currently enquire.c & its rules exist only for the benefit of people
writing new float-*.h files.  (This was not always the case.)  It's a
really, really, nasty piece of code, and anyway we don't have the newest
version. :-)  I suggest deleting it and its Makefile rules, and inserting
a note in some appropriate documentation ("How to write a new back end"?)
stating that 'enquire' might be a useful tool, and pointing to a website
with it.

What do others think?

--Nathanael