Auto merge of rust-lang#103649 - petrochenkov:docnotrait, r=GuillaumeGomez

rustdoc: Do not add external traits to the crate in `register_res`

It's not clear why it was done, and apparently it's no longer necessary now.
Such additions are unpredictable for early doc link resolution and would force us to collect all doc links from all external traits.

Fixes rust-lang#103463

Author: bors