Simplify timezone-handling code per proposal to pghackers: get rid of

Simplify timezone-handling code per proposal to pghackers: get rid of
setting timezone-related variables during transaction start.  They were
not used anyway in platforms that HAVE_TM_ZONE or HAVE_INT_TIMEZONE,
which it appears is *all* the platforms we are currently supporting.
For platforms that have neither, we now only support UTC or numeric-
offset-from-UTC timezones.