PERF: Remove redundant post_timings_summary index (#14164)

It's redundant since post_timings_unique exists which has a superset of
the columns with the same prefix.

db/migrate/20210824203421_remove_post_timings_summary_index.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class RemovePostTimingsSummaryIndex < ActiveRecord::Migration[6.1]
+  def change
+    remove_index :post_timings, column: [:topic_id, :post_number], name: :post_timings_summary, if_exists: true
+  end
+end