Revert "Simplify _Unqual template in core.time"

This reverts commit 3efbfd5731eb0dd1527bcbafee50619a976dfbd8.

The introduced change by #685 was not good because of the [bug 11722](https://d.puremagic.com/issues/show_bug.cgi?id=11722). And, `_Unqual` is used in the template constraints of operator overloading methods. So the change may cause regression on the `Duration` and `TickDuration` arithmetics.

So I'd like to just revert my change. Sorry.