Make pg_regress a tad simpler and more general-purpose by removing its

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by Joachim Wieland --- the rest of his patch is superseded by the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.