remove Monoid constraint on MonadState CPS.WriterT

[L0neGamer]

This is unncessary, so I'm suggesting we remove it.

[Kleidukos]

@L0neGamer I can't approve the decision for @sjshuck but I'd suggest you also add a changelog entry. :) Did you find it was redundant through a GHC warning?

[L0neGamer]

I found it was redundant because I recalled that the Monad instance for writer does not require Writer, and yet when using CPS.WriterT with a state monad and a writer that did not at that moment have a Monoid constraint, I got a compile time error when I shouldn't have.

for example:

adjustState :: WriterT w (State Int) ()
adjustState = modify' succ

shouldn't fail with CPS.WriterT.

There should be a warning for this, I'll dig it up (note: -Wredundant-constraints is turned on it seems but doesn't do anything here) and bump the changelog.

[sjshuck]

Thanks!