nfsd: remove nfsd4_prepare_cb_recall() declaration

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>