remove unnecessary type parameter foldMapIdentity

BTW, what's the reason there are [foldMapIdentity](https://github.com/scalaz/scalaz/blob/v7.0.0/core/src/main/scala/scalaz/Foldable.scala#L79) and [fold](https://github.com/scalaz/scalaz/blob/v7.0.0/core/src/main/scala/scalaz/Foldable.scala#L46)?
they have same signatures.