Remove completely unused concepts from codebase

Removes device_id and ClientInfo

device_id is never actually written, and the matrix.org DB has no
non-null entries for it. Right now, it's just cluttering up code.

This doesn't remove the columns from the database, because that's
fiddly.

tests/rest/client/v2_alpha/__init__.py

@@ -47,7 +47,6 @@ class V2AlphaRestTestCase(unittest.TestCase):
             return {
                 "user": UserID.from_string(self.USER_ID),
                 "admin": False,
-                "device_id": None,
                 "token_id": 1,
             }
         hs.get_auth().get_user_by_access_token = _get_user_by_access_token