How much money can be saved by removing seats?
The amount of time and manpower required to remove empty seats before a flight, transport them to the plane's next destination (presumably on another aircraft), then reinstall necessary seats for the next flight with 450 passengers (instead of this flight's 300 pax) would far exceed any potential cost savings of removing them for the flight. (Oh, don't forget, the remaining 350 seats would have to follow the plane to its 3rd destination because that flight is booked full.)
The biggest issue would be getting the seats themselves moved around so they could be reinstalled at the next destination. Not every aircraft uses the same seats, the mix of seats (cattle class, business class, first class) will be different on different versions of the same model of aircraft, even those owned by the same airline, so having a stock of seats at each airport that could be installed when the outgoing pax count is higher than the incoming pax count would be impractical.