These variables no longer have any useful purpose, since there's no reason
to special-case brute force timezones now that we have a valid
session_timezone setting for them.  Remove the variables, and remove the
SET/SHOW TIME ZONE code that deals with them.

The user-visible impact of this is that SHOW TIME ZONE will now show a
POSIX-style zone specification, in the form "<+-offset>-+offset", rather
than an interval value when a brute-force zone has been set.  While perhaps
less intuitive, this is a better definition than before because it's
actually possible to give that string back to SET TIME ZONE and get the
same behavior, unlike what used to happen.

We did not previously mention the angle-bracket syntax when describing
POSIX timezone specifications; add some documentation so that people
can figure out what these strings do.  (There's still quite a lot of
undocumented functionality there, but anybody who really cares can
go read the POSIX spec to find out about it.  In practice most people
seem to prefer Olsen-style city names anyway.)