image_info: Remove obsolete assert

The old code assumed the mip only had 1 layer thus a right overlap could not return mip 0. But with the new path we handle images that are both mip-mapped and multi-layer, thus this can happen

src/video_core/texture_cache/image_info.cpp

@@ -250,7 +250,6 @@ s32 ImageInfo::MipOf(const ImageInfo& info) const {
     if (mip < 0) {
         return -1;
     }
-    ASSERT(mip != 0);
 
     const auto mip_w = std::max(info.size.width >> mip, 1u);
     const auto mip_h = std::max(info.size.height >> mip, 1u);