Remove thumbnail generation from migrations

[maarcingebala]

One of the past PRs introduced thumbnail generation in the migration file. We had a discussion with @artursmet that this may cause performance issues if people have a large number of images in their projects. Instead, thumbnail generation should be triggered manually (e.g. with python manage.py create_thumbnails.)

Since we rather cannot remove the migration file as there are others that depend on it, I'd recommend simply removing the image generation code:

https://github.com/mirumee/saleor/blob/master/saleor/product/migrations/0077_generate_versatile_background_images.py

[stale]

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