staging: wilc1000: remove unused NM73131_0_BOARD

This patch removes the preprocessor definition, NM73131_0_BOARD which is
not used anymore.

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>