Remove environment-variable-based defaults in psql --help

Remove environment-variable-based defaults in psql --help

This seemed inconsistent with the --help output of other tools.
Depending on the values, it can cause ugly formatting.  Also, we're
not getting the defaults from libpq, we're just emulating the methods
libpq uses to derive these values, so they might not be 100% correct.

Author: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Discussion: https://www.postgresql.org/message-id/flat/50ca8ff35a8dd8f9ec89963b503571a7@oss.nttdata.com