Remove version(testStdDateTime) from SysTime.

It's no longer necessary and hasn't been for some time (it was to deal
with the fact that dmd ran out of memory on Windows when compiling
std.datetime's unit tests, which hasn't been a problem for a while). So,
we're removing it.
Remove version(testStdDateTime) from Date.
Remove version(testStdDateTime) from TimeOfDay.
Remove version(testStdDateTime) from DateTime.
Remove version(testStdDateTime) from std.datetime's interval types.
Remove the rest of version(testStdDateTime) from std.datetime.