Remove bundled delegate code.

Umbra currently uses a [specialized delegate module](https://github.com/libtcod/umbra/blob/170d4f111b2af08ec838366ec78bbec055643711/src/umbra/delegate.hpp) for callbacks.  I'm not really convinced that this code is useful post C++11, and I'd like to replace the uses of this with the more standard `std::function` type which is easier to work with.