[ObjC] Remove -[GPBFieldDescriptor optional]

As announced: https://protobuf.dev/news/2025-09-19/#objc-remove-apis

Removing the deprecated api as part of the ObjC Major Version bump
coming in 2026Q1.

PiperOrigin-RevId: 815768697

objectivec/GPBDescriptor.h

@@ -172,9 +172,6 @@ __attribute__((objc_subclassing_restricted))
 @property(nonatomic, readonly) GPBGenericValue defaultValue;
 /** Whether this field is required. Only valid for proto2 fields. */
 @property(nonatomic, readonly, getter=isRequired) BOOL required;
-/** Whether this field is optional. */
-@property(nonatomic, readonly, getter=isOptional) BOOL optional DEPRECATED_MSG_ATTRIBUTE(
-    "Check if fieldType is GPBFieldTypeSingle and that it is NOT required.");
 /** Type of field (single, repeated, map). */
 @property(nonatomic, readonly) GPBFieldType fieldType;
 /** Type of the key if the field is a map. The value's type is -dataType. */

objectivec/GPBDescriptor.m

@@ -630,13 +630,6 @@ uint32_t GPBFieldAlternateTag(GPBFieldDescriptor *self) {
   return (description_->flags & GPBFieldRequired) != 0;
 }
 
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wdeprecated-implementations"
-- (BOOL)isOptional {
-  return self.fieldType == GPBFieldTypeSingle && !self.isRequired;
-}
-#pragma clang diagnostic pop
-
 - (GPBFieldType)fieldType {
   GPBFieldFlags flags = description_->flags;
   if ((flags & GPBFieldRepeated) != 0) {