Move death voices from RenderInfantry into a separate trait

...named DeathSounds.

Note: I have some trouble moving SpawnCorpse into a separate trait as well (my attempt didn't even compile), but I consider the sounds far more important so I'm doing those first.

Infantry DeathSounds cannot be disabled with '-' on the desert shellmap since the ants' inheritance causes a bogus yaml removal crash in that case, so I had to set them to null instead.