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In a 4 wire system i.e. two hots, a neutral and a ground, and in a residential electrical system, you can derive both 240V and 120V from the same set of wires.
If you measure the voltage from either of the hots to the neutral, you will get 120V. Across the two hots and you have 240V because the two 120V sine waves are 180 degrees out of phase with each other. Think of it as +120V and -120V. The difference is 240V.
Your old stove and you new oven uses the 240V for the heating elements and probably uses the 120V for the controls, lights etc. The new cook top only needs 240V so therefore no neutral is required.
I hope that explains it for you. Let me know if you need more help.
You can either disconnect the white at the panel or do what most people would do and simply wire nut and tape the neutral conductor at the outlet box. As it is right now, you can touch the neutral. It carries no voltage or current. It is safe but you want to tape it up in the box so that it doesn't make contact with any other wire and cause a short.
This is done all the time and completely within code.