vulkan: Remove gsk_vulkan_image_new_from_data()

vulkan: Remove gsk_vulkan_image_new_from_data()

This is unused now that all the code uses map/unmap.

The only thing that map/unmap doesn't do that the old code did, was use
a staging image instead as alternative to a staging buffer for image
uploads.

However, that code is not necessary for anything, so I'm sure we can do
without.