Remove injury slowdown

[Fuzzy 'Mc-Cones' Mcnomnoms]

Many of the zombies are already quite fast or have the ability to slow you down. It's already hard enough to get away from them if you're caught by them, getting hit slows you down to a point where it's nearly impossible to get away. Removing the movement penalty from getting hurt, or perhaps limiting it to below ~30% hp makes it a lot easier for survivors to get away while not making it too much harder for zombies.