rfc20: drop the attributes section

Problem: the Rv1 attributes section is used by the early Fluxion
queue implementation to store the assigned queue name in the
system.scheduler key, but queues are now handled differently per
RFC 33.

Drop the attributes section.

Author: garlick

spec_20.rst

@@ -234,30 +234,6 @@ R Format
   occurs when a new Flux instance is started cannot do the same for the opaque
   :data:`scheduling` key.
 
-.. data:: attributes
-
-  The purpose of the :data:`attributes` key is to provide optional
-  information on this *R* document. The :data:`attributes` key SHALL
-  be a dictionary of one key: :data:`system`.
-
-  Other keys are reserved for future extensions.
-
-  .. data:: system
-
-    Attributes in the :data:`system` dictionary provide additional system
-    information that have affected the creation of this *R* document.
-    All of the system attributes are optional.
-
-    A common system attribute is:
-
-    .. describe:: scheduler
-
-      The value of the :data:`scheduler` key is a free-from dictionary that
-      may provide the information specific to the scheduler used
-      to produce this document. For example, a scheduler that
-      manages multiple job queues may add ``queue=batch``
-      to indicate that this resource set was allocated from within
-      its ``batch`` queue.
 
 Example R
 =========