rustdoc: do not filter out cross-crate `Self: Sized` bounds
All type parameters **except `Self`** are implicitly `Sized` ([via](https://doc.rust-lang.org/nightly/std/marker/trait.Sized.html)). Previously, we disregarded the exception of `Self` and omitted cross-crate `Sized` bounds of *any* type parameter *including* `Self` when rendering.
From now on, we *do* render cross-crate `Self: Sized` bounds.
Most notably, in `std` we now finally properly render the `Sized` bound of the `Clone` trait as well as the `Self: Sized` bound on `Iterator::map`.
Fixes#24183.
``@rustbot`` label T-rustdoc A-cross-crate-reexports
r? rustdoc
28a1bafd48