nfsd: remove nfsd4_prepare_cb_recall() declaration

[ Upstream commit 18224dc5 ]

nfsd4_prepare_cb_recall() has been removed since
commit 0162ac2b ("nfsd: introduce nfsd4_callback_ops"),
so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>