Remove this check for object-safety during selection of trait object …

…candidates

I don't really understand what it's for, but see the comment here:
rust-lang#50173 (comment)

where arielb1 said

> Does this check do anything these days? I think `$0: Trait` is always considered ambiguous

and nikomatsakis agreed we may be able to get rid of it

src/librustc/traits/select.rs

@@ -2111,17 +2111,6 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                     data.principal().with_self_ty(this.tcx(), self_ty)
                 }
                 ty::Infer(ty::TyVar(_)) => {
-                    // Object-safety candidates are only applicable to object-safe
-                    // traits. Including this check is useful because it helps
-                    // inference in cases of traits like `BorrowFrom`, which are
-                    // not object-safe, and which rely on being able to infer the
-                    // self-type from one of the other inputs. Without this check,
-                    // these cases wind up being considered ambiguous due to a
-                    // (spurious) ambiguity introduced here.
-                    let predicate_trait_ref = obligation.predicate.to_poly_trait_ref();
-                    if !this.tcx().is_object_safe(predicate_trait_ref.def_id()) {
-                        return;
-                    }
                     debug!("assemble_candidates_from_object_ty: ambiguous");
                     candidates.ambiguous = true; // could wind up being an object type
                     return;