I don't think you need to do that at all, the Slim is designed to handle the heat load of the Xbox just fine. If the fan has 4 wires it uses what's called PWM to control the fan speed and will adjust it's speed according to the heat load of the xbox. If it has three wires it uses voltage regulation to change speeds.
I would advise against cutting monitor wire as it will cause you xbox to probably log a fan fail event, which may be readable by XBL. If you insist upon doing it, look the fan connector on the mother board. With the alignment tab at the bottom ( like this [_]) the pin that is at the left side will be the control/monitor pin. Cut the wire that plugs into it.
NOTE: I take NO RESPONSIBILITY WHATSOEVER for your xbox. You break it, your fault.