What should I do with a 3-way light switch that is no longer needed?
You can't really remove the wire from the 3-way in the kitchen without tearing the wallboard out and removing it that way.  It's probably stapled above the boxes that it goes to or from.  

You first need to determine if the receptacle is being fed from the 3-way.  If so then you might have to connect 2 of the wires that feed from the kitchen 3-way.  If the receptacle is being fed from someplace else then you still might have to connect the wires in the kitchen 3-way or open the receptacle up and remove the wires from the kitchen 3-way and then change the the other 3-way to a single pole.  The kitchen 3-way can have a blank cover put on it or you can buy a switch - blank plate or whatever configuration you need.  Make sure you turn the power off when working around the wires and when the power is on use a non-contact tester to determine what wires are hot.  When the power is on, keep one hand in your back pocket.