My intuition is that removing a pickup would not reduce sustain. The magnets are pulling slightly on the strings, which would increase sustain, but not enough to be noticeable.

It really comes down to the wood of your guitar. You can feel the vibration of the guitar when you play, and the difference between one with a lot of sustain vs one without is pretty noticeable.

The pickups and amp, and gain levels also make a difference. The pickups on my guitar are extremely sensitive, so any little touch is quite loud through my amp.