[nmh-workers] Content caching code

[Ken Hornstein]

As long as we're getting rid of stuff ...

Does anyone use the content caching code?  (See mhstore(1) for details).
First, it turns out that when mhn was split into mhstore, mhshow, and
mhlist, there was never a call to cache_all_messages() (the entry point
into mhcachesbr.c) put in those individual programs, so in essence it
would only ever store something in a cache if you called mhn (which has
been deprecated for approximately forever).  There are some calls to
find_cache() in mhparse.c, but since there is nothing in the normal case
that stores something in the cache it would never be used.  Also, the
security properties of a public cache seem pretty scary to me in this
day and age (if you used one, and since the code mostly seems to not
work I doubt anyone does, but I've been proven wrong before).

I propose we get rid of it all; clearly no one has missed it.  Thoughts?

--Ken