pgsql: Arrange to remove pg_default_acl entries completely if their ACL

Log Message:
-----------
Arrange to remove pg_default_acl entries completely if their ACL setting
is changed to match the hard-wired default. This avoids accumulating useless
catalog entries, and also provides a path for dropping the owning role without
using DROP OWNED BY. Per yesterday's complaint from Jaime Casanova, the
need to use DROP OWNED BY for that is less than obvious, so providing this
alternative method might save some user frustration.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
alter_default_privileges.sgml (r1.4 -> r1.5)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/alter_default_privileges.sgml?r1=1.4&r2=1.5)
pgsql/src/backend/catalog:
aclchk.c (r1.165 -> r1.166)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/aclchk.c?r1=1.165&r2=1.166)