[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>

lib/python/qmk/cli/__init__.py

@@ -58,7 +58,6 @@ subcommands = [
     'qmk.cli.generate.keyboard_c',
     'qmk.cli.generate.keyboard_h',
     'qmk.cli.generate.keycodes',
-    'qmk.cli.generate.keycodes_tests',
     'qmk.cli.generate.keymap_h',
     'qmk.cli.generate.make_dependencies',
     'qmk.cli.generate.rgb_breathe_table',