Is it common, or even correct, for both wall switches in a double gang box to provide power to a ceiling fan/light fixture?
I believe you are correct in assuming that one switch controlled the fan and one controlled the lights originally.

My guess is that the wiring at the fan just has both the red and black wires connected to the line side of the wireless receiver, which would let either switch turn the fan/light on.

I would look at the wiring at the fan to confirm this. If I am correct, then yes, you should be able to disconnect the red wire. I would disconnect it from the fan and from the switch in the box and cap both ends.