NFS: Remove unused function nfs_umount

nfs_umount() has been unused since 2013's
commit 4580a92d ("NFS: Use server-recommended security flavor by
default (NFSv3)")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20250218215250.263709-1-linux@treblig.org


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