Dusk Rat Morph / Illusionary Form fix

Added a FALSE return for CanMorphTo() for Dusk Rats. This was an
oversight when it was added - this monster has an incredibly strong
poison that players can add to their attacks by abusing Illusionary
Form, a level 3 Riija spell that otherwise has only marginal effects.
Using the Dusk Rat illusion causes arrows to proc the poison, which can
stack up to 4 times to do 40 damage a tick, typically ending a battle
all on its own in just a few seconds.

Furthermore, Reflections will also assume the Dusk Rat form and proc the
poison as well. Reflections also being level 3 Riija means exponential
abuse, by spamming Reflections that spam super poisons.

There are even further ways to break this tactic by adding in other
spells and behaviors, but let's just eliminate all of it by eliminating
Dusk Rats as a morph choice.