Remove version(testStdDatetime) from std.datetime.

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.

This results in an annoyingly large diff, but it's mostly indentation changes due to the `version` blocks being removed, so if you put ?=w1 on the end of the url for the diffs, the diffs should then be visible (at least the individual diffs will be - I don't know about the total diff, since it's still a large number of lines affected). On the bright side, while the diffs are tedious to look over, at least they're very straightforward. Sorry for having to deal with this, but it's well past time that these extraneous lines be removed.