staging: r8188eu: RfOnOffDetect is unused

Remove the RfOnOffDetect function, which is not used.

Suggested-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211010175204.24029-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>