GTimeZone: remove some dead code

Remove code that parsed out and stored the unused isstd and isgmt
fields.
GObject: tweak property deprecation warnings

Don't emit property deprecation warnings for construct properties that
are being set to their default value during construction, but _do_ emit
them in all cases when the property was explicitly given to
g_object_new().

https://bugzilla.gnome.org/show_bug.cgi?id=732184