Strictly speaking of productivity without justifying Trump’s plan in any way - labor shortage increases productivity. Cheap labor decreases productivity. Expensive labor forces capitalists to invest in new equipment and training to be able to produce the same output with fewer labor hours. Simple example - fastening the same number of bolts using manual screwdrivers in more hands vs electric screwdrivers in fewer hands.

Outside of that, removing a significant portion of the population at any one time would be significantly disruptive in many regards.