staging: r8188eu: remove the remaining usb endpoint functions

Remove the last bunch of unused RT_usb_endpoint_... functions.

Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210910202151.4007-5-martin@kaiser.cx


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