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

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 ed10a3faae,
it was used on `Sized`, and that usage is gone.

compiler/rustc_span/src/symbol.rs

@@ -934,7 +934,6 @@ symbols! {
         panic_unwind,
         panicking,
         param_attrs,
-        parent_trait,
         partial_cmp,
         partial_ord,
         passes,