Remove unused type variable

The PureScript compiler warns about unused type variables as of 0.7.6.

Author: tfausak

src/Optic/Laws/Lens.purs

@@ -8,7 +8,7 @@ module Optic.Laws.Lens where
   import Prelude (Eq, (&&), (==))
 
   -- | A valid `Lens` satisfies all three of the following laws.
-  validLens :: forall s a b. (Eq a, Eq s) => LensP s a -> s -> a -> a -> a -> Boolean
+  validLens :: forall s a. (Eq a, Eq s) => LensP s a -> s -> a -> a -> a -> Boolean
   validLens l s x y z = getSet l s
                      && setGet l s x
                      && setSet l s y z