staging: r8188eu: remove unused hal_xmit_handler define

hal_xmit_handler is not used by the r8188eu driver. Remove it.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Acked-by: Pavel Skripkin <paskripkin@gmail.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20230109212852.75612-8-martin@kaiser.cx


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