Do I need to dismantle the ballast on a metal halide bulb fixture before replacing with an LED bulb?
LED light bulbs generally need to be connected to the mains directly, assuming they are designed for 120/240 Volts AC.

LED light bulbs do not need a ballast, a ballast is only needed for gas discharge lamps like Metal halide and fluorescent lamps. You might even damage a LED light by leaving the ballast in place.

So yes, you need to remove it and connect the 120/240 V AC LED bulb directly to the mains.