nfs: no PG_private waiters remain, remove waker

Since commit 4f52b6bb ("NFS: Don't call COMMIT in ->releasepage()"),
no tasks wait on PagePrivate, so the wake introduced in commit 95905446
("NFS: avoid deadlocks with loop-back mounted NFS filesystems.") can
be removed.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>