Remove PGINTERVALSTYLE from the set of special environment variables for

Remove PGINTERVALSTYLE from the set of special environment variables for
libpq.  As noted by Peter, adding this variable created a risk of unexpected
connection failures when talking to older server versions, and since it
doesn't do anything you can't do with PGOPTIONS, it doesn't seem really
necessary.  Removing it does occasion a few extra lines in pg_regress.c,
but saving a getenv() call per libpq connection attempt is perhaps worth
that anyway.