Breaking Change: Remove deprecated FieldDescriptor::is_optional() in OSS. Use (!is_required() && !is_repeated()) instead

https://protobuf.dev/news/2025-09-19/#cpp-remove-apis

PiperOrigin-RevId: 843767334

Author: anandolee

src/google/protobuf/descriptor.h

@@ -940,9 +940,6 @@ class PROTOBUF_EXPORT FieldDescriptor : private internal::SymbolBase,
   bool is_required() const;
   bool is_repeated() const;  // Whether or not the field is repeated/map field.
 
-  ABSL_DEPRECATE_AND_INLINE()
-  bool is_optional() const;  // Use !is_required() && !is_repeated() instead.
-
   bool is_packable() const;  // shorthand for is_repeated() &&
                              //               IsTypePackable(type())
   bool is_map() const;       // shorthand for type() == TYPE_MESSAGE &&
@@ -2939,10 +2936,6 @@ inline FieldDescriptor::CppStringType FieldDescriptor::cpp_string_type() const {
   return static_cast<FieldDescriptor::CppStringType>(cpp_string_type_);
 }
 
-inline bool FieldDescriptor::is_optional() const {
-  return !is_repeated() && !is_required();
-}
-
 inline bool FieldDescriptor::is_repeated() const {
   ABSL_DCHECK_EQ(is_repeated_, static_cast<Label>(label_) == LABEL_REPEATED);
   return is_repeated_;