When `xtimer_on_ztimer` is used, `xtimer` does not need to perform any clock conversion. Especially undefined macros of `XTIMER_HZ` and `XTIMER_SHIFT` should no longer result in compilation issues, as they are not used internally anyway.
When `xtimer_on_ztimer` is used, `xtimer` does not need to perform any clock conversion. Especially undefined macros of `XTIMER_HZ` and `XTIMER_SHIFT` should no longer result in compilation issues, as they are not used internally anyway.