The spring is in there to keep the throttle barrel against the the guide pin as it rolls open. If it was removed, the barrel and low needle would float in and out making the engine lean and rich accordingly. The fact that this spring moves the throttle open on it's own is the nature of the beast but don't remove the spring.
This is why an external return spring is a good idea, but it must not interfere with the internal springs ability to keep the barrel out against the guide pin.
I believe this is one of the biggest reasons guys have problems with engine tuning-- because of an improperly set up throttle linkage.
If the servo linkage pushes the barrel in as it is rolled open or closed, it changes the mixture curve and the engine will never run consistently. It's not such an issue with helis as both ends usually have ball links so there is no side loading unless the canopy or something is rubbing the rod, but it is very important on an airplane where the rod usually rubs the firewall and can push in on the barrel if the rod isn't bent correctly.

Kirk