NFS: remove unused time_delta field from struct nfs_server

The last code that was using this was removed via commit ca0daa27
("NFS: Cache aggressively when file is open for writing") which was
merged in v4.8-rc1, so it can be removed completely.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Link: https://lore.kernel.org/r/20250613094439.82338-3-ailiop@suse.com


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