dashboard/app: fix rendering of dup bugs caption

After addition of collapsible sections on the bug page,
we show 2 captions for the duplicate bugs table.
One for the collapsible span "Duplicate bugs (2)",
and another inside for the table "duplicates (2):".
Remove the inner duplicate caption.

Author: dvyukov

dashboard/app/main.go

@@ -1898,7 +1898,6 @@ func loadDupsForBug(c context.Context, bug *Bug, state *ReportingState,
 	}
 	group := &uiBugGroup{
 		Now:         timeNow(c),
-		Caption:     "duplicates",
 		ShowPatched: true,
 		ShowStatus:  true,
 		Bugs:        results,