git.loader: Migrate away from SWHConfig mixin

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

Test Plan

tox

and docker:

time doco exec swh-loader swh loader run git https://github.com/huge-success/sanic.git
INFO:swh.loader.git.BulkLoader:Load origin 'https://github.com/huge-success/sanic.git' with type 'git'
Enumerating objects: 143, done.
Counting objects: 100% (143/143), done.
Compressing objects: 100% (102/102), done.
Total 15402 (delta 78), reused 77 (delta 40), pack-reused 15259
INFO:swh.loader.git.BulkLoader:Listed 1072 refs for repo https://github.com/huge-success/sanic.git
{'status': 'eventful'}
docker-compose exec swh-loader swh loader run git   0.49s user 0.39s system 1% cpu 48.581 total

---

Migrated from D4128 (view on Phabricator)