staging: r8188eu: remove free_xmit_priv field from struct hal_ops

Remove free_xmit_priv function pointer field from struct hal_ops definition
in include/hal_intf.h, as it is now no longer used anywhere in the driver.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210818234253.208271-7-phil@philpotter.co.uk


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