virtio: Revert "virtio: add helper virtio_find_vqs_ctx_size()"

This reverts commit fe3dc04e: the
API is now unused and in fact can't be implemented on top of a legacy
device.

Fixes: fe3dc04e ("virtio: add helper virtio_find_vqs_ctx_size()")
Cc: "Xuan Zhuo" <xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220816053602.173815-3-mst@redhat.com>