Revert "add index to failed jobs" (#6739)

This reverts commit 36281b285a.

database/migrations/0001_01_01_000002_create_jobs_table.php

@@ -42,8 +42,6 @@ return new class extends Migration
             $table->longText('payload');
             $table->longText('exception');
             $table->timestamp('failed_at')->useCurrent();
-
-            $table->index(['connection', 'queue', 'failed_at']);
         });
     }