[Merge] ~muktupavels/unity:compizconfig into unity:master

[Alberts Muktupāvels]

> I think we no longer need 00_remove_obsolete_plugins_in_unity_session_v3.py.
> 
> We do not support upgrades from versions before Ubuntu 18.04, and 18.04 users
> already have this migration applied.

Removed commit that added this file.
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https://code.launchpad.net/~muktupavels/unity/+git/unity/+merge/362759
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