You won't notice a problem at all by pulling that air pump off. Your catalyst will become less efficient possibly, but I hardly think that is an issue on a 1983 bus. You may well not even have a cat convertor under there.
Air pumps are a sneaky device. The formal statement of their purpose is that they provide additional oxygen to encourage oxidation (burning) in the manifolds and downpipes and to allow a 3-way converter to store extra oxygen for later use. The reality is that air pumps existed to dilute the exhaust so that the parts per million a sniffer picked up dropped below the threshold.
Modern electronic air pumps do a much better job of mimicking the formal purpose.