You can leave the pump in place but to disable the pump you will need to remove the large nylon gear inside the intake manifold. You will also need to cap off the oil injection nozzles. Leave the oil tank in place and the oil line feeding the rotary gear and the return line in place.
I've grown weary of trying to talk people out of switching to premix, I think there are enough threads here for people to make their own decision.
I don't know what to tell you. You don't need both. Is there oil in the tank? You need oil in the tank to lubricate the rotary valve gear, and if the pump is still functioning to lubricate the pump.
Well the only way, that I know, to be sure if the oil pump is still functioning is to remove the carbs., you'll see two small nozzles in the throat of the intake manifold. Remove the spark plugs, ground the spark plug cables, turn the engine over and see if there's oil coming out of the nozzles.
I really don't know why so many were determined to bypass the oil injection. I swear on my kids, in the years that I serviced these things (1990-1997) I have never seen one of these fail. It was usually the guys who raced or the ones who just had to have their special, shiny aftermarket intake/rotary cover that simply defaulted to pre-mix.
I have tested it by grounding the wires, reaching down and moving the pump lever while turning the engine over, without moving throttle so that only oil will be delivered.
Then removing the rotary cover (leaving the carbs attached, cables and hoses) by taking out the 4 bolts that hold it on, then getting a look at the injection ends. The rotary valve should be good and oiled up on the surface too. I just found it easier on those 720's to get those 4 cover bolts on and off (one tricky one) than it was to remove the carbs and such. Heck, I used to remove the rotary cover to get the carbs off anyways.
So, if you're saying that the oil injection is working and you have premix in the tank, you can just add fresh gas to the premix and go ahead and run it. But it is important that you verify that the oil injection is working.