Code action: remove redundant constraints

[hsenag]

Should be a good first issue if identifying the right bit of code to remove is simple enough (as weith haskell/ghcide#54).

If the -Wredundant-constraints warning is on, GHC emits warnings like:

C:\msys64\home\ganesh\temp\x\Foo.hs:3:1: warning:
    • Redundant constraint: Ord a
    • In the type signature for:
           foo :: forall a. Ord a => a -> Int

Unfortunately the source range covers all of the type signature for foo, not just the redundant constraint itself.