remove \DateTime check for very old PHP versions

Author: ramunasd

PhpAmqpLib/Wire/AMQPAbstractCollection.php

@@ -241,9 +241,6 @@ protected function encodeValue($val)
             $val = $this->encodeVoid();
         } elseif ($val instanceof \DateTimeInterface) {
             $val = array(self::T_TIMESTAMP, $val->getTimestamp());
-        } elseif ($val instanceof \DateTime) {
-            // PHP <= 5.4 has no DateTimeInterface
-            $val = array(self::T_TIMESTAMP, $val->getTimestamp());
         } elseif ($val instanceof AMQPDecimal) {
             $val = array(self::T_DECIMAL, $val);
         } elseif ($val instanceof self) {