doc: nrf: wifi: remove incorrect doxygen group

We cannot render/link Doxygen groups that belong to another project like
Zephyr, see nrfconnect#19341 for
progress on that.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/nrf/libraries/networking/wifi_credentials.rst

@@ -66,5 +66,3 @@ API documentation
 
 | Header file: :file:`include/zephyr/net/wifi_credentials.h`
 | Source files: :file:`subsys/net/lib/wifi_credentials`
-
-.. doxygengroup:: wifi_credentials