`Drain` should not be marked as `must_use`

`Drain` is currently marked as `#[must_use = "iterators are lazy and do nothing unless consumed"]`. However, while `drain` can be used to iterate, it is also commonly used to just remove a range of characters.

Fwiw, none of the `Drain` implementations in `std` (that I've looked at) are marked as `must_use` either, presumably for that very reason.