Add overload to set_difference_by_key(...) that doesn't require values2

First of all, great library!

I am having some confusion in understanding the role of `values_first2` in `set_difference_by_key` as mentioned [here](https://thrust.github.io/doc/group__set__operations_ga1ea27dd891f59cb73f6f28f8e97bfb1c.html).

In general terms, the result of set difference `D = A - B` will not contain values from `B` therefore `values_first2` can never be part of `D`. So `values_first2` will never be used in this function and thus should be removed as an input argument. Is my understanding correct? Thanks.