[nrf fromtree] tests/subsys/settings/functional/fcb: Remove native_posix overlay

native_posix is being removed as it has reached its end of life in 4.2,
after being deprecated since 4.0.

So this overlay will not serve any purpose anymore.

Note native_sim did not have an overlay of its own.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Signed-off-by: Sebastian Głąb <[email protected]>
(cherry picked from commit db62135)

Author: aescolar