svn.loader: Migrate away from SWHConfig mixin

Related to swh-core#1532 (closed) Related to swh-core#1410 Related to swh-core!159 (closed)

Test Plan

tox

and

docker run (with docker-compose.override.yml):

time doco exec swh-loader swh loader run svn svn://scm.gforge.inria.fr/svnroot/fdf/
INFO:swh.loader.svn.SvnLoader:Load origin 'svn://scm.gforge.inria.fr/svnroot/fdf/' with type 'svn'
INFO:swh.loader.svn.SvnLoader:Processing revisions [1-922] for {'swh-origin': 'svn://scm.gforge.inria.fr/svnroot/fdf/', 'remote_url': 'svn://scm.gforge.inria.fr/svnroot/fdf', 'local_url': b'/tmp/swh.loader.svn.gytutrft-204/fdf', 'uuid': b'0491c1f1-070f-0410-b5d3-808586ddf757'}
{'status': 'eventful'}
docker-compose exec swh-loader swh loader run svn   0.58s user 0.48s system 0% cpu 4:29.62 total

---

Migrated from D4126 (view on Phabricator)