Rollup merge of #88875 - notriddle:notriddle/cleanup-unused-trait-selection, r=Mark...

cleanup(rustc_trait_selection): remove vestigial code from rustc_on_unimplemented

This isn't allowed by the validator, and seems to be unused.
When it was added in ed10a3faae1fd1176b2edf4a61438e0542c103b9,
it was used on `Sized`, and that usage is gone.

diff --cc compiler/rustc_span/src/symbol.rs
Simple merge