vdpa/mlx5: Remove unused function declarations

Commit 29064bfd

 ("vdpa/mlx5: Add support library for mlx5 VDPA implementation")
declared but never implemented these.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Message-Id: <20230803143041.23388-1-yuehaibing@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>