Further fix pg_upgrade crossversion test for adminpack.
The DROP DATABASE step needs an "if exists" option, as the oldest
branches we test don't have the contrib_regression_adminpack DB.
Also remove unnecessary command to drop the extension from the
regression database; no version has installed it there during
buildfarm testing.
Discussion: https://postgr.es/m/
0CFB76D0-0510-48B2-9916-
1199F93BC28C@yesql.se