users: don't log mod actions for account upgrades.

Account upgrades are now logged on the /user_upgrades page, so they
no longer need to be recorded as mod actions. The mod actions log should
be reserved for privileged actions performed by Builders and above. They
also tended to spam the mod actions log.

test/unit/user_upgrade_test.rb

@@ -31,12 +31,6 @@ class UserUpgradeTest < ActiveSupport::TestCase
             assert_equal("complete", @user_upgrade.status)
           end
 
-          should "log an account upgrade modaction" do
-            assert_difference("ModAction.user_account_upgrade.count") do
-              @user_upgrade.process_upgrade!("paid")
-            end
-          end
-
           should "send the recipient a dmail" do
             assert_difference("@user_upgrade.recipient.dmails.received.count") do
               @user_upgrade.process_upgrade!("paid")