Remove extra set_error_handler callback argument

It is unused and would cause an error on PHP ≥ 8.0:
https://www.php.net/manual/en/function.set-error-handler.php#refsect1-function.set-error-handler-parameters

Not sure if the handler is even necessary – it was introduced in 175196d6c2 but I did not manage to reproduce the original error (Entity 'nbsp' not defined). It was probably fixed by f2a43b476c.

(based on commit da755013aa)

tests/ReadabilityTest.php

@@ -325,7 +325,7 @@ class ReadabilityTest extends \PHPUnit\Framework\TestCase
     }
 
     // dummy function to be used to the next test
-    public function error2Exception($code, $string, $file, $line, $context)
+    public function error2Exception($code, $string, $file, $line)
     {
         throw new \Exception($string, $code);
     }