Fix or remove the /api/queues endpoint

[msm-code]

Looks like this endpoint doesn't work anymore:

ERROR:karton.dashboard.app:Exception on /api/queues [GET]
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1951, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1820, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1949, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1935, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/karton/dashboard/app.py", line 196, in get_queues_api
    for identity, queue in state.queues.items()
  File "/usr/local/lib/python3.7/site-packages/karton/dashboard/app.py", line 196, in <dictcomp>
    for identity, queue in state.queues.items()
  File "/usr/local/lib/python3.7/site-packages/karton/dashboard/app.py", line 100, in to_dict
    "tasks": sorted(tasks, key=lambda t: t.last_update, reverse=True),
  File "/usr/local/lib/python3.7/site-packages/karton/dashboard/app.py", line 100, in <lambda>
    "tasks": sorted(tasks, key=lambda t: t.last_update, reverse=True),
AttributeError: 'str' object has no attribute 'last_update'

It went undetected pretty long, so I assume it's not used by anyone. We can probably safely remove it instead of fixing it (but I have no hard stance on this).

I wish we could e2e test this, but this looks like a pain to setup test environment for.