This reverts commit aaddf6be7f as it broke
handling of dates when local time zone is BST, whose offset not counting DST
is 0, as for UTC, but which still should be handled as local timezone,
see #17220.
With the current wxDateTime handling of time zones, FromTimezone(Local)
doesn't make much sense anyhow, so abandon attempts to try making it work as
to really do it we need to specify the time zone being converted from too, as
explained in the second point of #10445.
See #16585.
f6d9d7962e