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.

synapse/handlers/admin.py

@@ -34,6 +34,7 @@ class AdminHandler(BaseHandler):
 
         d = {}
         for r in res:
+            # Note that device_id is always None
             device = d.setdefault(r["device_id"], {})
             session = device.setdefault(r["access_token"], [])
             session.append({