remove unnecessary restriction on join

src/func.rs

@@ -191,7 +191,6 @@ pub trait Monad: Applicative {
     /// Equivalent of Haskell's `join`.
     fn join<'a, A: 'a>(ffa: Self::F<'a, Self::F<'a, A>>) -> Self::F<'a, A>
     where
-        Self::F<'a, A>: 'a,
         Self: 'a,
     {
         Self::bind(ffa, |fa| fa)