kconfig: Fix nrf91 NONSECURE dependency

It is expressed that the BOARD depends on whether NONSECURE is enabled
or not. But it is the other way around. Depending on the selected
board, it may or may not be possible to enable/disable NONSECURE.

The dependency is going in the wrong direction, this reversed edge is
observed to be able to create a cycle in the dependency graph.

Fix the dependency by removing it.

It is left as future work to enforce that enabling/disabling NONSECURE
is done in a way that is compatible with selecting
BOARD_NRF9160_PCA10090 vs BOARD_NRF9160_PCA10090NS.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>