safe transmute: don't mark user impls as unambiguous

ref: https://github.com/rust-lang/rust/pull/92268#discussion_r925243794

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -884,7 +884,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         if obligation.has_param_types_or_consts() {
-            candidates.ambiguous = false;
             return;
         }