[pg15] test: untrack PgIndexBackfillTest.CreateIndexSimultaneously

Summary:
According to issue #21824, this test is flaky on master.  It is also
flaky on pg15.  Remove it from pg15_tests/passing_tests.tsv.

Test Plan: Jenkins: skip

Reviewers: aagrawal, tfoucher

Reviewed By: tfoucher

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D34437

pg15_tests/passing_tests.tsv

@@ -231,7 +231,6 @@ pgwrapper_pg_get_lock_status-test	PgGetLockStatusTestRF3.*
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.ColocatedRetainDeleteMarkers
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.ColocatedRetainDeleteMarkersRecovery
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.ColocatedRetainDeleteMarkersRecoveryViaSeveralRequests
-pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.CreateIndexSimultaneously
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.InsertsWhileCreatingIndexDisableWait
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.InsertsWhileCreatingIndexEnableWait
 pgwrapper_pg_index_backfill-test	PgIndexBackfillTest.Large