Remove unused references to internal::DownCast (#19372)

Closes #19372

COPYBARA_INTEGRATE_REVIEW=#19372 from Tradias:remove-downcast f126025
PiperOrigin-RevId: 700363447

Author: Tradias

src/google/protobuf/extension_set_unittest.cc

@@ -52,7 +52,6 @@ extern bool fully_verify_message_sets_opt_out;
 
 namespace {
 
-using ::google::protobuf::internal::DownCast;
 using TestUtil::EqualsToSerialized;
 
 // This test closely mirrors google/protobuf/compiler/cpp/unittest.cc

src/google/protobuf/map_test.inc

@@ -217,7 +217,6 @@ struct MapTestPeer {
 };
 
 namespace {
-using internal::DownCast;
 
 // Map API Test =====================================================
 

src/google/protobuf/message_lite.h

@@ -1200,11 +1200,6 @@ T* OnShutdownDelete(T* p) {
   return p;
 }
 
-inline void AssertDownCast(const MessageLite& from, const MessageLite& to) {
-  ABSL_DCHECK(TypeId::Get(from) == TypeId::Get(to))
-      << "Cannot downcast " << from.GetTypeName() << " to " << to.GetTypeName();
-}
-
 template <bool test_call, typename MessageLite>
 PROTOBUF_ALWAYS_INLINE MessageLite* MessageCreator::PlacementNew(
     const MessageLite* prototype_for_func,

src/google/protobuf/repeated_field_reflection_unittest.inc

@@ -22,7 +22,6 @@ namespace protobuf {
 namespace {
 
 
-using internal::DownCast;
 using UNITTEST::ForeignMessage;
 using UNITTEST::TestAllExtensions;
 using UNITTEST::TestAllTypes;