[guix] make guix-clean remove also var and output

Summary: and remove the complex precious_dir machinery. Our guix-clean script is not using it, it is much simpler than the one used by Core.

Test Plan:

contrib/guix/guix-clean
contrib/guix/guix-build

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15643