Don't add DST offset when converting to local time zone.

Local time zone already logically includes DST, even if its offset doesn't
account for it (because the offset depends on the date, so it can't be part of
TZ itself), so don't add it again when converting to/from it.

Closes #16585.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77899 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775