Simplify and merge unwanted-module drop logic in AdjustUpgrade.pm.

Simplify and merge unwanted-module drop logic in AdjustUpgrade.pm.

In be7800674 and followups, we failed to notice that there was
already a better way to do it: instead of using DROP DATABASE
IF EXISTS, we can check the list of existing DBs.  Also, there
seems no reason not to merge this into the pre-existing code
for getting rid of unwanted module databases.

Discussion: https://postgr.es/m/1066872.1710006597@sss.pgh.pa.us