configs: j721s2: merge HS and non-HS defconfigs

K3 devices have runtime type board detection. Make the default defconfig
include the secure configuration. Then remove the HS specific config.

Non-HS devices will continue to boot due to runtime device type detection.
If TI_SECURE_DEV_PKG is not set the build will emit warnings, for non-HS
devices these can be ignored.

Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Acked-by: Andrew Davis <afd@ti.com>