Remove redundant CREATEUSER/NOCREATEUSER options in CREATE ROLE et al.

Once upon a time we did not have a separate CREATEROLE privilege, and
CREATEUSER effectively meant SUPERUSER.  When we invented CREATEROLE
(in 8.1) we also added SUPERUSER so as to have a less confusing keyword
for this role property.  However, we left CREATEUSER in place as a
deprecated synonym for SUPERUSER, because of backwards-compatibility
concerns.  It's still there and is still confusing people, as for example
in bug #13694 from Justin Catterson.  9.6 will be ten years or so later,
which surely ought to be long enough to end the deprecation and just
remove these old keywords.  Hence, do so.