TRT-2848: Drop unused idx_test_daily_totals_date index

pkg/db/migrations/000012_drop_test_daily_totals_date_index.down.sql

@@ -0,0 +1,2 @@
+CREATE INDEX IF NOT EXISTS idx_test_daily_totals_date
+    ON test_daily_totals (date);

pkg/db/migrations/000012_drop_test_daily_totals_date_index.up.sql

@@ -0,0 +1,7 @@
+-- idx_test_daily_totals_date exists only to make an unscoped
+-- SELECT MAX(date) FROM test_daily_totals fast across ~3,700 partitions.
+-- Nothing queries MAX(date) without a release filter anymore
+-- (dailysummary.MaxSummaryDate and cumulativesummary.MaxDailySummaryDate
+-- were the last two callers), so this index is now pure write overhead:
+-- every insert into test_daily_totals maintains it for no reader.
+DROP INDEX IF EXISTS idx_test_daily_totals_date;

pkg/db/migrations/MANIFEST

@@ -18,3 +18,4 @@
 000009_add_lifecycle_to_prow_job_run_tests
 000010_drop_test_analysis_by_job_by_dates
 000011_add_lifecycle_to_summaries
+000012_drop_test_daily_totals_date_index