Drop other index behind pg_upgrade test issue.

Fix the test failure by dropping the index in question.  Missed by
commit 57ae788.

Per buildfarm member crake.

Backpatch: 11-12 only

Author: petergeoghegan

src/test/regress/expected/btree_index.out

@@ -313,6 +313,7 @@ select reloptions from pg_class WHERE oid = 'btree_idx1'::regclass;
 (1 row)
 
 drop index btree_idx1;
+drop index btree_idx_err;
 --
 -- Test for multilevel page deletion
 --

src/test/regress/sql/btree_index.sql

@@ -141,6 +141,7 @@ create index btree_idx_err on btree_test(a) with (vacuum_cleanup_index_scale_fac
 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;
+drop index btree_idx_err;
 
 --
 -- Test for multilevel page deletion