also remove the comments about logging ID which is already removed (#4822)

Author: charlotteliang

Firebase/InstanceID/FIRInstanceIDCheckinService.m

@@ -210,9 +210,6 @@ - (NSDictionary *)checkinParametersWithExistingCheckin:
   NSInteger userNumber = 0;        // Multi Profile may change this.
   NSInteger userSerialNumber = 0;  // Multi Profile may change this
 
-  // This ID is generated for logging purpose and it is only logged for performance
-  // information for backend, not secure information.
-  // TODO(chliang): Talk to backend team to see if this ID is still needed.
   NSString *timeZone = [NSTimeZone localTimeZone].name;
   int64_t lastCheckingTimestampMillis = checkinPreferences.lastCheckinTimestampMillis;