Removing reactive

I'm leaving it in the list because it's such a classic FRP library that people are bound to look for it. But I'm marking it as "unmaintained": I can't include an example for it because it doesn't compile on a recent GHC.

I don't think it has even compiled since at least GHC 6.12; it expects category-extras-0.53.5, which defines its own Monad instance for Either, which conflicts with every version of base between base-4.3.0.0 (GHC 7.0.1) and base-4.6.0.1 (GHC 7.6.3).