[z1851] remove no more needed CORPSE_FALLING state, move FallGround method to more appropriate place

Note: the Mutate(new EffectMovementGenerator(0)); line does not apply to 1.x

(based on commit aab5e05)
(based on commit a29ee64)

(cleaned version of 7edb5aa)

(based on commit [s1412] - a16afa9)