cmake: remove deprecated `set_conf_file()` support The `set_conf_file()` was deprecated in 6d4ba3490fee518da9136ce54d45d82200e800c6 which is before Zephyr v1.14 LTS. Let's remove the support now, before releasing a new LTS. Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>