staging: wilc1000: remove define WILC_AP_EXTERNAL_MLME and ifdef line

This driver use WILC_AP_EXTERNAL_MLME define always. Delete define
WILC_AP_EXTERNAL_MLME and ifdef line.

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