staging: wilc1000: remove paltform define PLAT_WMS8304

This patch removes PLAT_WMS8304 and it's related codes as well. We will not
use this way of supporting other platform. This will be supported if necessary
by device tree later.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>