nfs: remove nfs_wait_atomic_killable() and nfs_write_prepare() declaration

nfs_write_prepare() has been removed since
commit a4cdda59 ("NFS: Create a common pgio_rpc_prepare
function"), so remove it.

nfs_wait_atomic_killable() has been removed since
commit 723c921e ("sched/wait, fs/nfs: Convert wait_on_atomic_t()
usage to the new wait_var_event() API"), so remove it.

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