fs: remove NFFS subsystem

NFFS is removed as it has serious bugs (by design) which haven't
been resolved since extended range of time.
One of most serious issues bunch were described here:
https://github.com/apache/mynewt-nffs/issues/10



Since lack of support NFFS upsterem it doesn't make sense to keep
it in zephyr.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>