rapi: Remove “/2” resource, deprecate “/”

These were never really useful. Neither gave a complete list of
available resources—the documentation in doc/rapi.rst is much better at
that.

Since some monitoring code might use the “/” resource it's kept around
but will only return a JSON “null”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>