pgsql: Make pg_regress a tad simpler and more general-purpose by

Log Message:
-----------
Make pg_regress a tad simpler and more general-purpose by removing its
code to forcibly drop regressuser[1-4] and regressgroup[1-2].  Instead,
let the privileges.sql test do that for itself (this is made easy by
the recent addition of DROP ROLE IF EXISTS).  Per a recent patch proposed
by Joachim Wieland --- the rest of his patch is superseded by the
rewrite into C, but this is a good idea we should adopt.

Modified Files:
--------------
    pgsql/src/test/regress:
        pg_regress.sh (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/pg_regress.sh.diff?r1=1.64&r2=1.65)
    pgsql/src/test/regress/expected:
        privileges.out (r1.33 -> r1.34)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/privileges.out.diff?r1=1.33&r2=1.34)
    pgsql/src/test/regress/sql:
        privileges.sql (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/privileges.sql.diff?r1=1.17&r2=1.18)