Drop other index behind pg_upgrade test issue.

Peter Geoghegan <pg@bowt.ie>

Commit: 28cf4d93d045b9862ef2d8b49a533bd2912228c1
Author: Peter Geoghegan <pg@bowt.ie>
Date: 2021-03-11T07:32:55Z
Releases: 11.12
Drop other index behind pg_upgrade test issue.

Fix the test failure by dropping the index in question.  Missed by
commit 57ae7885.

Per buildfarm member crake.

Backpatch: 11-12 only

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