imx: remove mx1ads defconfig

For some time now the defconfig for mx1ads produces a kernel for
a versatile board.  As CONFIG_ARCH_MX1ADS is included in the mx1
defconfig just get rid of mx1ads_defconfig.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <kernel@pengutronix.de>