Fix PotionEffect removal.

The method in EntityLiving to remove a potion effect was remapped during
the 1.7.5 update.  The method invocation in CraftLivingEntity was not
updated to invoke the remapped method, which has led to a random method in
LivingEntity being called in its place.

This commit corrects the behavior of removePotionEffect by changing the
method to invoke the remapped method as opposed to EntityLiving#m(float).

Thanks to @gabizou for finding this issue.