Remove TZ environment-variable entry from postgres reference page.

Remove TZ environment-variable entry from postgres reference page.

The server hasn't paid attention to the TZ environment variable since
commit ca4af308c32d03db, but that commit missed removing this documentation
reference, as did commit d883b916a947a3c6 which added the reference where
it now belongs (initdb).

Back-patch to 9.2 where the behavior changed.  Also back-patch
d883b916a947a3c6 as needed.

Matthew Somerville