staging: r8188eu: remove rtw_free_pwrctrl_priv prototype

The rtw_free_pwrctrl_priv has already been removed in commit 6729e754
("staging: r8188eu: _free_pwrlock is empty"). Remove the prototype as well.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210918141141.28547-1-martin@kaiser.cx


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