Remove DATETZ type

[hawkfish]

The ICU extension could be extended to handle DATE WITH TIME ZONE by converting the date_t values to UDates. The rule should be that the given number of days is added to the epoch.

One possible problem here is that ICU likes to left shift negative values, which triggers the address sanitiser. We either need a version of the code that fixes this problem or some hack for dates prior to the epoch.

[hawkfish]

When I added this I was under the mistaken impression that the standard defined DATE WITH TIME ZONE. It does not; the only other TZ type is TIME, which is largely useless. This bug is now about removing the type from the code.