NFS: remove unused pnfs_ld_data field from struct nfs_server

The last code that was using this was removed via commit 20d655d6
("pnfs/blocklayout: use the device id cache") which was merged in
v3.18-rc1, so it can be removed completely.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Link: https://lore.kernel.org/r/20250613094439.82338-4-ailiop@suse.com


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