model: kill Origin.type attribute

it was still here for bw-compat but should not be necessary any more.

Test Plan

Depends on !89 (closed).

---

Migrated from D2815 (view on Phabricator)

swh/model/hypothesis_strategies.py

@@ -75,7 +75,6 @@ def timestamps_with_timezone():
 def origins():
     return builds(
         Origin,
-        type=sampled_from(['git', 'hg', 'svn', 'pypi', 'deb']),
         url=urls())