Remove the "extra checkout paths" feature in order to simplify the
configuration file, and thus also the upcoming web interface to edit it.
The "extra checkout paths" feature was added to aid in transition from
the Flamenco v2 shaman system to the v3 system. It is very unlikely that
there is still use of Flamenco v2 by people who will want to migrate to
v3 in the future. I expect that if they wanted to, they'd have done so
by now.
Ref: #104403
This is not yet working, it's just a direct copy of the Manager of Flamenco
2, with Logrus replaced by Zerolog. The API has been documented in
flamenco-manager.yaml as a starting point for the integration.
