pgsql: Drop index behind pg_upgrade test issue.

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 9f3665fb.

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/57ae7885cfe7a6301e1fd4b59c2e525ecc35cd55

Modified Files
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src/test/regress/expected/btree_index.out | 1 +
src/test/regress/sql/btree_index.sql | 1 +
2 files changed, 2 insertions(+)