man: drop /var/spool/ mention from file-hierarchy(7) man page

man: drop /var/spool/ mention from file-hierarchy(7) man page

Today it seems this is mostly used by mail and printer servers, and it's
not clear to me at all what the property is that makes
/var/spool/<package> the better place for the relevant data than
/var/lib/<package>.

Hence, in the interest of shortening the spec, let's not mention the dir
anymore. In particular as the dir really isn't used by us much, for
example we do not have a counterpart for RuntimeDirectory=,
StateDirectory=, … that would cover the spool.

Since most systems these days we care about probably come *without* a
printer or mail server, let's maybe no mention this in the man page that
is supposed to discuss the rough skeleton how things are set up. After
all, people are supposed to exend the skeleton with their stuff, and
this sounds more like a case for an extension of the skeleton instead of
being considered part of the skeleton itself.