Cleanup some NFSv2 remains

Signed-off-by: Amit Dror <amitd@infinidat.com>
Remove unused FSAL_DIGEST_SIZEOF option

Signed-off-by: Amit Dror <amitd@infinidat.com>
Remove FSAL_DIGEST_FILEID3 and FSAL_DIGEST_FILEID4 options

These are not needed anymore.
Changed readdir/readdirplus to get the fileid directly from the
FSAL object attributes.

Signed-off-by: Amit Dror <amitd@infinidat.com>