remove 'method_exists' clause

src/php/lib/Grpc/AbstractCall.php

@@ -114,14 +114,9 @@ abstract class AbstractCall
     protected function _serializeMessage($data)
     {
         // Proto3 implementation
-       if (method_exists($data, 'serializeToString')) {
         return $data->serializeToString();
     }
 
-        // Protobuf-PHP implementation
-        return $data->serialize();
-    }
-
     /**
      * Deserialize a response value to an object.
      *