Remove -[RACSignal promiseOnScheduler:]

The _intended_ use of promises is for [true memoization](http://stackoverflow.com/questions/14066651/how-to-using-reactivecocoa-to-transparently-authenticate-before-making-api-calls/14072445#14072445)—performing a calculation (or action) exactly once, and then always using that same result thereafter. Promises are _not_ meant to bring back replaying, so we should remove the API that makes it easy to convert a cold signal into one.

Resolves #971.