Removed HOTPLUG=1 from the monitor hotplug rules.
Since udev didn't understand this key, it wasn't using it anyway.
This gets rid of the confusing error message in /var/log/messages.
TEST=Plugged/unplugged a VGA monitor and made sure modes still switched.
...also, built with FEATURES=keepwork and checked this file:
...validated that the unknown key was actually being ignored by udev, so this doesn't change behavior (other than getting rid of the error message).