enhance: remove redundant indices from submission_defs table

improves submission throughput by 20% without any impact on select queries

Benchmark:
250_questions form with 20K existing submissions
JMeter parameters:
300 thread
60 test duration
1500 per 10 sec target throughput
release 40 threads in batch

Result:
95/sec throughput with 4% error without this change
115/sec throughput without any error with this change

lib/model/migrations/20230321-01-optimize-indices-sub-defs.js

@@ -0,0 +1,25 @@
+// Copyright 2023 ODK Central Developers
+// See the NOTICE file at the top-level directory of this distribution and at
+// https://github.com/getodk/central-backend/blob/master/NOTICE.
+// This file is part of ODK Central. It is subject to the license terms in
+// the LICENSE file found in the top-level directory of this distribution and at
+// https://www.apache.org/licenses/LICENSE-2.0. No part of ODK Central,
+// including this file, may be copied, modified, propagated, or distributed
+// except according to the terms contained in the LICENSE file.
+
+const up = async (db) => {
+  await db.raw('DROP INDEX public.submission_defs_createdat_index;');
+  await db.raw('DROP INDEX public.submission_defs_current_index;');
+  await db.raw('DROP INDEX public.submission_defs_id_submissionid_index;');
+  await db.raw('DROP INDEX public.submission_defs_submissionid_index;');
+};
+
+const down = async (db) => {
+  await db.raw('CREATE INDEX submission_defs_createdat_index ON public.submission_defs USING btree ("createdAt");');
+  await db.raw('CREATE INDEX submission_defs_current_index ON public.submission_defs USING btree (current);');
+  await db.raw('CREATE INDEX submission_defs_id_submissionid_index ON public.submission_defs USING btree (id, "submissionId");');
+  await db.raw('CREATE INDEX submission_defs_submissionid_index ON public.submission_defs USING btree ("submissionId");');
+};
+
+module.exports = { up, down };
+