staging: wlan-ng: Remove unused function pointer typedef freebuf_method_t

Remove the 'freebuf_method_t' typedef as it is not used.

Signed-off-by: Chen Lin <chen.lin5@zte.com.cn>
Link: https://lore.kernel.org/r/1613447646-4678-1-git-send-email-chen45464546@163.com


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