dmails: don't save copies of outgoing dmails sent by DanbooruBot.

There's not much sense in saving copies of everything DanbooruBot sends
in DanbooruBot's inbox. They probably won't be checked so it just bloats
the dmails table.

test/unit/user_test.rb

@@ -31,12 +31,11 @@ class UserTest < ActiveSupport::TestCase
         bot = FactoryGirl.create(:user)
         Danbooru.config.stubs(:system_user).returns(bot)
 
-        assert_difference("Dmail.count", 2) do
+        assert_difference("Dmail.count", 1) do
           @user.promote_to!(User::Levels::GOLD)
         end
 
         assert(@user.dmails.exists?(from: bot, to: @user, title: "You have been promoted"))
-        assert(bot.dmails.exists?(from: bot, to: @user, title: "You have been promoted"))
       end
     end