Remove a function_exists() check for error_get_last()

It was only relevant until we dropped support for PHP < 5.2
diff --git a/system/core/Common.php b/system/core/Common.php
index c008bd5..00e3030 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -634,7 +634,7 @@
 	 */
 	function _shutdown_handler()
 	{
-		$last_error = function_exists('error_get_last') ? error_get_last() : NULL;
+		$last_error = error_get_last();
 		if (isset($last_error) &&
 			($last_error['type'] & (E_ERROR | E_PARSE | E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_COMPILE_WARNING)))
 		{