staging: r8188eu: remove unused defines and enums

Remove a couple of unused defines and an unused enum
from rtl8188e_cmd.h.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211020195401.12931-5-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>