Drop index behind pg_upgrade test issue.

The vacuum_cleanup_index_scale_factor storage parameter was set in a
btree index that was previously left behind in the regression test
database.  As a result, the index gets tested within pg_dump and
pg_restore tests, as well as pg_upgrade testing.  This won't work when
upgrading to Postgres 14, though, because the storage parameter was
removed on that version by commit 9f3665f.

Fix the test failure by dropping the index in question.

Per buildfarm member crake.

Discussion: https://postgr.es/m/CAH2-WzmeXYBWdhF7BMhNjhq9exsk=E1ohqBFAwzPdXJZ1XDMUA@mail.gmail.com
Backpatch: 11-12 only

Author: petergeoghegan

src/test/regress/expected/btree_index.out

@@ -312,6 +312,7 @@ select reloptions from pg_class WHERE oid = 'btree_idx1'::regclass;
  {vacuum_cleanup_index_scale_factor=70.0}
 (1 row)
 
+drop index btree_idx1;
 --
 -- Test for multilevel page deletion
 --

src/test/regress/sql/btree_index.sql

@@ -140,6 +140,7 @@ create index btree_idx_err on btree_test(a) with (vacuum_cleanup_index_scale_fac
 -- Simple ALTER INDEX
 alter index btree_idx1 set (vacuum_cleanup_index_scale_factor = 70.0);
 select reloptions from pg_class WHERE oid = 'btree_idx1'::regclass;
+drop index btree_idx1;
 
 --
 -- Test for multilevel page deletion