Update / clean up remaining aufs-related bits

Originally created by @intrigeri on #17451 (Redmine)

We should git grep -i aufs and either update or clean up each occurrence.

For example, aufs support in the IUK generation code can go away, and IIRC we have some kludges to workaround aufs bugs somewhere.

Feature Branch: feature/8415-overlayfs+force-all-tests

Edited May 15, 2020 by intrigeri

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