Remove sum example;

examples/sum.lua

@@ -1,13 +0,0 @@
-local Rx = require 'rx'
-
--- Uses the 'sum' combinator to sum all values produced by the observable and
--- dumps the resulting output.
-local sumObservable = Rx.Observable.create(function(observer)
-  observer:onNext(1)
-  observer:onNext(2)
-  observer:onNext(3)
-  observer:onNext(4)
-  observer:onCompleted()
-end)
-
-sumObservable:sum():dump('sum')