Remove the seats and its empty weight would change as well as possibly C of G moments! That would bugger up your loading calculations as well as C of G calculations as the datum would change from the published datum for the aircraft.
Rubbish, if you remove the seat and don't put anything on the floor in that area then it will not bugger up anything, I don't recall the exact weight of a 172 seat but assuming (not that you ever should in aviation) it is 10 kgs maximum, just do the W&B calculation with a weight of -10kgs for that arm. Simples.
Removing a seat in itself is never going to kill anyone, what might kill you however is the vast amount of gear that you can get in that area should it not be suitably tied down.