NFSv4: remove nfs4_renewd_prepare_shutdown() declaration

nfs4_renewd_prepare_shutdown() has been removed since
commit 3050141b ("NFSv4: Kill nfs4_renewd_prepare_shutdown()"),
so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>