Improve some codes according to the reviews

- improve diagnostics of field uniqueness check and representation check
- simplify the implementation of field uniqueness check
- remove some useless codes and improvement neatness

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -181,11 +181,6 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
         | ty::Ref(_, _, Mutability::Not)
         | ty::Array(..) => Err(NoSolution),
 
-        // Check for anonymous adts.
-        ty::Adt(adt, generics) if adt.is_anonymous() => {
-            Ok(adt.non_enum_variant().fields.iter().map(|f| f.ty(ecx.tcx(), generics)).collect())
-        }
-
         ty::Dynamic(..)
         | ty::Str
         | ty::Slice(_)