[Core] use keycode_string in unit tests (#25042)

* tests: use keycode_string feature

With a proper keycode to string implementation in qmk there is no need
to use the unit tests only implementation anymore.

Signed-off-by: Stefan Kerkmann <karlk90@pm.me>

* tests: remove keycode_util feature

This feature is no longer used as we switched the tests to the
keycode string implementation.

Signed-off-by: Stefan Kerkmann <karlk90@pm.me>

tests/combo/test_combo.cpp

@@ -3,7 +3,6 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include "keyboard_report_util.hpp"
-#include "quantum.h"
 #include "keycode.h"
 #include "test_common.h"
 #include "test_driver.hpp"