boards, sys: purge feuerware_radios

* `feuerware_radios` is unused, remove from `radio.h` header
* FEUERWARE_CONF_NUM_RADIOS is superfluous without it, remove all occurrences
Merge pull request #1881 from LudwigOrtmann/purge_feuerware_radio

boards, sys: purge feuerware_radios