NFS: Fix the notifications when renaming onto an existing file

NFS appears to be returning an unnecessary "delete" notification when
we're doing an atomic rename. See

  http://bugzilla.gnome.org/show_bug.cgi?id=575684

The fix is to get rid of the redundant call to d_delete().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>