Removal of deprecated views pg_user, pg_group, pg_shadow

Greetings,

The question of removing the pre-role, deprecated, views of pg_user,
pg_group and pg_shadow has come up again.

I figured a new thread was in order, however, to allow others to weigh
in on it.

Note that these views have not been consistently maintained and have
ended up including some role attributes from recent versions (eg:
bypassrls) but were missed when others were added (eg: createrole).
There are properly maintained and cared for role-based versions of all
of these views, which are pg_roles, pg_auth_members, and pg_authid,
respectively.

As we move forward with the other many changes in PG10, it seems like a
good time to remove these inconsistent and ancient views that were
introduced when roles were added in 2005.

Thanks!

Stephen