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 ed10a3f,
it was used on `Sized`, and that usage is gone.

Author: notriddle

compiler/rustc_span/src/symbol.rs

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

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -154,9 +154,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                 flags.push((sym::from_method, Some(method.to_string())));
             }
         }
-        if let Some((t, _)) = self.get_parent_trait_ref(&obligation.cause.code) {
-            flags.push((sym::parent_trait, Some(t)));
-        }
 
         if let Some(k) = obligation.cause.span.desugaring_kind() {
             flags.push((sym::from_desugaring, None));