Drop dmail filters.

Few people used dmail filters (~900 users in 5 years) and even fewer
used them correctly. Most people used them to try to block dmail spam,
but usually they either blocked too much (by adding common words that
are present in nearly all dmails, causing all mails to them to be
filtered) or too little (blocking specific email addresses or urls,
which usually are never seen again after the spammer is banned).
Nowadays the spam detection system does a better job of filtering spam.

test/functional/maintenance/user/dmail_filters_controller_test.rb

@@ -1,35 +0,0 @@
-require "test_helper"
-
-module Maintenance
-  module User
-    class DmailFiltersControllerTest < ActionDispatch::IntegrationTest
-      context "The dmail filters controller" do
-        setup do
-          @user1 = create(:user)
-          @user2 = create(:user)
-        end
-
-        context "update action" do
-          setup do
-            as(@user1) do
-              @dmail = create(:dmail, owner: @user1)
-            end
-          end
-
-          should "not allow a user to create a filter belonging to another user" do
-            params = {
-              :dmail_id => @dmail.id,
-              :dmail_filter => {
-                :words => "owned",
-                :user_id => @user2.id
-              }
-            }
-
-            put_auth maintenance_user_dmail_filter_path, @user1, params: params
-            assert_not_equal("owned", @user2.reload.dmail_filter.try(&:words))
-          end
-        end
-      end
-    end
-  end
-end