relaxed `?Sized` bound getting implied by super trait warn

[lcnr]

the following code should lint that the ?Sized bound is unused:'

trait Proj {
    type Assoc: Clone + ?Sized;
}

fn foo<T: ?Sized + Clone>() {}

we imply Sized as it's a super trait of Clone, so the ?Sized bound does not do anything.

cc @davidtwco, related to sized_hierarchy migration stuff