panvk: drop an obsolete assert of explicit mod plane count

It has become obsolete since the proper support of the external
multi-planar images:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35299

Test;
- no abort in camera and video use with debug build panvk

Reviewed-by: Christoph Pillmayer <christoph.pillmayer@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36959>

src/panfrost/vulkan/panvk_image.c

@@ -179,7 +179,6 @@ panvk_image_get_explicit_mod(
    assert(image->vk.samples == 1);
    assert(image->vk.array_layers == 1);
    assert(image->vk.image_type != VK_IMAGE_TYPE_3D);
-   assert(explicit->drmFormatModifierPlaneCount == 1);
    assert(panvk_image_can_use_mod(image, mod));
 
    return mod;