nfscl: Do not do readahead for directories

For a very long time, the NFS client has done readahead for
directory blocks. Unlike data blocks, the readahead cannot
begin until the Readdir RPC reply has been received, since
the directory offset cookie in that Readdir RPC reply is needed.
As such, the readahead is serialized and does not seem to
provide any real benefit.

Recent testing/benchmarking shows that removing this
readahead code for Readdir does not have a negative impact
on performance.

Therefore, this patch deletes the readahead code for Readdir,
which simplifies the code and may make future changes simpler.

MFC after: 1 month