fix: no generate with default assoc item

[A4-Tacks]

• And remove bounds for associated type

Example

trait $0Foo {
    type Item: ?Sized;

    const N: usize = 3;
}

Before this PR

impl<T: ?Sized> Foo for $0T {
    type Item: ?Sized;

    const N: usize = 3;
}

After this PR

impl<T: ?Sized> Foo for $0T {
    type Item;
}