Remove callback function from autodoc unit test.

It doesn't seem to belong there at all, there is a dedicated callback unit
test for it and it was added to the initial version of autodoc.i back in
124253d698 without any explanation, so just
remove it.

As this callback was used in a PHP test, perform this test for callback.i now
and use "%(uppercase)s" construct inside %callback to test that this works.

Examples/test-suite/php/callback_runme.php

@@ -0,0 +1,9 @@
+<?php
+
+require "tests.php";
+require "callback.php";
+// In 2.0.6 and earlier, the constant was misnamed.
+if (gettype(callback::FOO_I_Cb_Ptr) !== 'resource') die("callback::foo_T<int> not a resource\n");
+
+check::done();
+?>