tst_QDateTime: remove silly test for timezone date too early

Let's not try to to compare our QTimeZone handling with the system one.
Our handling goes beyond the range of the POSIX APIs, so that's a recipe
for error.

Change-Id: Iee8cbc07c4434ce9b560ffff13ca4a4f335bdbae
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>