Remove redundant lifetime bound from impl Borrow for Cow

The lifetime bound `B::Owned: 'a` is redundant and doesn't make a difference,
because `Cow<'a, B>` comes with an implicit `B: 'a`, and associated types
will outlive lifetimes outlived by the `Self` type (and all the trait's
generic parameters, of which there are none in this case), so the implicit `B: 'a`
implies `B::Owned: 'a` anyway.

The explicit lifetime bound here does however end up in documentation,
and that's confusing in my opinion, so let's remove it ^^

library/alloc/src/borrow.rs

@@ -21,7 +21,6 @@ use Cow::*;
 impl<'a, B: ?Sized> Borrow<B> for Cow<'a, B>
 where
     B: ToOwned,
-    <B as ToOwned>::Owned: 'a,
 {
     fn borrow(&self) -> &B {
         &**self