tests/subsys/fs/fs_api: remove dir-multi-open test

That whether a FS implementation allow to open
same directory multiple time is FS implementation
dependent. It shouldn't be tested. Such test was removed.

However need to test whether API transfers failures from
underlying FS properly.

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