Remove check for None-keys in _check_valid_key.
Remove check for None-keys in _check_valid_key.

None-keys should not get to _check_valid_key() anyways, so we should not treat them there.

karteek/simplekv,fmarczin/simplekv,karteek/simplekv,mbr/simplekv,mbr/simplekv,fmarczin/simplekv