staging: r8188eu: remove rtw_set_rpwm

After we dropped the call to set HW_VAR_SET_RPWM, the rtw_set_rpwm
function doesn't do anything useful.

Remove the function and the rpwm component of struct pwrctrl_priv,
which is not used outside of rtw_set_rpwm.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210913185110.3065-2-martin@kaiser.cx


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