Don't try to render auto-trait bounds with any inference variables

Don't try to render auto-trait bounds with any inference variables

Previously, we checked if the target of a projection type was itself an
inference variable. However, for Rustdoc rendering purposes, there's no
distinction between an inference variable ('_') and a type containing
one (e.g. (MyStruct<u8, _>)) - we don't want to render either of them.

Fixes #60269

Due to the complexity of the original bug, which spans three different
crates (hyper, tower-hyper, and tower), I have been unable to create a
minimized reproduction for the issue.