Remove utility functions which are already defined in the new Prelude

[eayus]

Duplicate definitions of new prelude functions resulted in ambiguous usage error.

Since the Prelude now contains a functor implementation for Pair, the Data.Tuple.mapSnd could also be removed in favour of using the functor map, but left it in since it may be preferred due to its more verbose name.

Note this branch does not compile even after these changes, due to some unrelated errors.

[clayrat]

Thanks!