Add remove transducer

[jasonjckn]

Can we add the 'remove' transducer, it's defined in clojure @ https://github.com/clojure/clojure/blob/master/src/clj/clojure/core/reducers.clj#L167

It's just the dual to 'Filter', but often leads to more readability, e.g. Remove(isnothing), i can send a PR if you'd like... pretty trivial of course.