warp7: secure_defconfig: Remove secure_defconfig

This patch removes warp7_secure_defconfig. A previous patch set
CONFIG_ARMV7_BOOT_SEC_DEFAULT=y on the unsecure WaRP7 config. Fabio asked
if I could confirm that the NXP and upstream kernels will boot on the WaRP7
with CONFIG_ARMV7_BOOT_SEC_DEFAULT=y. I can confirm that this is the case,
so there's no need to support the secure defconfig - drop it now.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Suggested-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>