arch posix: Cleanup old code

When the native simulator use was introduced,
the POSIX architecture and SOC were split in 2 versions:

One was the old version, which remained used by native_posix and the
other NATIVE_APPLICATION based targets.
The new version was a shim on top of the native simulator threading
and CPU start/stop emulation.

This was done to ensure no regressions were introduced in the old
targets while the native simulator was tested and matured.

The old SOC code was removed a small while after, and all
NATIVE_APPLICATION targets moved to use the shim version on top of the
native simulator.

Now we remove also the old arch code, so native_posix and its
NATIVE_APPLICATION kin, also use the native simulator NCT component
instead.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>