nfs: no PG_private waiters remain, remove waker

nfs: no PG_private waiters remain, remove waker

Since commit b9fb98c6d825 ("NFS: Don't call COMMIT in ->releasepage()"),
no tasks wait on PagePrivate.

Thus the wake introduced in commit 673b89688cfc ("NFS: avoid deadlocks
with loop-back mounted NFS filesystems.") can be removed.

Link: http://lkml.kernel.org/r/20170103182234.30141-2-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>