Remove unused let bindings in generated Haskell code
The Haskell Code generated from FlatCurry sometimes contains unnecessary let-bindings like
let s2 = s1
in ...
It would be good if those bindings would be removed to simplify the generated code.
Since the computation of new supplies was quite fiddly, it may be simpler to implement a kind of simplification for AbstractHaskell, which, for example, could replace/inline simple let-bindings.