NFS: Remove unused FLUSH_SYNC support in nfs_initiate_pgio()

NFS: Remove unused FLUSH_SYNC support in nfs_initiate_pgio()

If the FLUSH_SYNC flag is set, nfs_initiate_pgio() will currently
wait for completion, and then return the status of the I/O operation.
What we actually want to report in nfs_pageio_doio() is whether or
not the RPC call was launched successfully, whereas actual I/O
status is intended handled in the reply callbacks.

Since FLUSH_SYNC is never set by any of the callers anyway, let's
just remove that code altogether.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>