[Messaging] Remove extraneous const on FIRMessagingIsAPNSSyncMessage (#13245)

Author: andrewheard

FirebaseMessaging/Sources/FIRMessaging.m

@@ -60,7 +60,7 @@
 
 NSString *const FIRMessagingErrorDomain = @"com.google.fcm";
 
-const BOOL FIRMessagingIsAPNSSyncMessage(NSDictionary *message) {
+BOOL FIRMessagingIsAPNSSyncMessage(NSDictionary *message) {
   if ([message[kFIRMessagingMessageViaAPNSRootKey] isKindOfClass:[NSDictionary class]]) {
     NSDictionary *aps = message[kFIRMessagingMessageViaAPNSRootKey];
     if (aps && [aps isKindOfClass:[NSDictionary class]]) {