pgsql: Simplify and merge unwanted-module drop logic in AdjustUpgrade.p

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/519443162dce52a3b10372338401d63084f720b8

Modified Files
--------------
src/test/perl/PostgreSQL/Test/AdjustUpgrade.pm | 19 +++++++------------
1 file changed, 7 insertions(+), 12 deletions(-)