[ObjC] Remove some of the legacy generated formats support

Remove some of the legacy union fields from the structs used for
the generated code. The library has already declared it doesn't
support these old formats (via the version check done by the
C pre processor).

PiperOrigin-RevId: 718846100

objectivec/GPBDescriptor_PackagePrivate.h

@@ -89,13 +89,8 @@ typedef struct GPBMessageFieldDescription {
   // C identifier for large apps.
   const char *name;
   union {
-    // className is deprecated and will be removed in favor of clazz.
-    // kept around right now for backwards compatibility.
-    // clazz is used iff GPBDescriptorInitializationFlag_UsesClassRefs is set.
-    char *className;  // Name of the class of the message.
-    Class clazz;      // Class of the message.
-    // For enums only.
-    GPBEnumDescriptorFunc enumDescFunc;
+    Class clazz;                         // Class of the message.
+    GPBEnumDescriptorFunc enumDescFunc;  // Function to get the enum descriptor.
   } dataTypeSpecific;
   // The field number for the ivar.
   uint32_t number;
@@ -134,29 +129,12 @@ typedef NS_OPTIONS(uint8_t, GPBExtensionOptions) {
 typedef struct GPBExtensionDescription {
   GPBGenericValue defaultValue;
   const char *singletonName;
-  // Before 3.12, `extendedClass` was just a `const char *`. Thanks to nested
-  // initialization
-  // (https://en.cppreference.com/w/c/language/struct_initialization#Nested_initialization) old
-  // generated code with `.extendedClass = GPBStringifySymbol(Something)` still works; and the
-  // current generator can use `extendedClass.clazz`, to pass a Class reference.
   union {
-    const char *name;
     Class clazz;
   } extendedClass;
-  // Before 3.12, this was `const char *messageOrGroupClassName`. In the
-  // initial 3.12 release, we moved the `union messageOrGroupClass`, and failed
-  // to realize that would break existing source code for extensions. So to
-  // keep existing source code working, we added an unnamed union (C11) to
-  // provide both the old field name and the new union. This keeps both older
-  // and newer code working.
-  // Background: https://github.com/protocolbuffers/protobuf/issues/7555
   union {
-    const char *messageOrGroupClassName;
-    union {
-      const char *name;
-      Class clazz;
-    } messageOrGroupClass;
-  };
+    Class clazz;
+  } messageOrGroupClass;
   GPBEnumDescriptorFunc enumDescriptorFunc;
   int32_t fieldNumber;
   GPBDataType dataType;