Migrate away from examining the syntax of a proto file via reflection.

PiperOrigin-RevId: 511792205

java/core/src/main/java/com/google/protobuf/DynamicMessage.java

@@ -535,11 +535,10 @@ public final class DynamicMessage extends AbstractMessage {
           fields.clearField(oldField);
         }
         oneofCases[index] = field;
-      } else if (field.getFile().getSyntax() == Descriptors.FileDescriptor.Syntax.PROTO3) {
-        if (!field.isRepeated()
-            && field.getJavaType() != FieldDescriptor.JavaType.MESSAGE
-            && value.equals(field.getDefaultValue())) {
-          // In proto3, setting a field to its default value is equivalent to clearing the field.
+      } else if (!field.hasPresence()) {
+        if (!field.isRepeated() && value.equals(field.getDefaultValue())) {
+          // Setting a field without presence to its default value is equivalent to clearing the
+          // field.
           fields.clearField(field);
           return this;
         }