Remove SIMPLEPIE_PHP5 constant

This was used for compatibility functions, and was an internal constant.
If you're using this in your own code, it's probably broken by now
anyway. :)

SimplePie/Core.php

@@ -372,11 +372,6 @@
  */
 define('SIMPLEPIE_IANA_LINK_RELATIONS_REGISTRY', 'http://www.iana.org/assignments/relation/');
 
-/**
- * Whether we're running on PHP5
- */
-define('SIMPLEPIE_PHP5', version_compare(PHP_VERSION, '5.0.0', '>='));
-
 /**
  * No file source
  */