Remove the DELETEs from pg_shadow and pg_group that pg_dumpall used to

Remove the DELETEs from pg_shadow and pg_group that pg_dumpall used to
emit when given the --clean option, in favor of individual DROP ROLE
commands.  The old technique could not possibly work in 8.1, and was
never a very good idea anyway IMHO.  The DROP ROLE approach has the
defect that the DROPs will fail for roles that own objects or have
privileges, but perhaps we can improve that later.