DEV: Remove migration that is no longer required.

Avoid creating the index twice.

Follow-up to 628319aad3

db/migrate/20200813090811_create_partial_index_on_post_search_data.rb

@@ -1,17 +0,0 @@
-# frozen_string_literal: true
-
-class CreatePartialIndexOnPostSearchData < ActiveRecord::Migration[6.0]
-  disable_ddl_transaction!
-
-  def up
-    execute <<~SQL
-    CREATE INDEX CONCURRENTLY idx_regular_post_search_data ON post_search_data USING GIN(search_data) WHERE NOT private_message
-    SQL
-  end
-
-  def down
-    execute <<~SQL
-    DROP INDEX IF EXISTS idx_regular_post_search_data;
-    SQL
-  end
-end