remove functionality to delete helix by clicking in side view

[dave-doty]

At first this was the only way to delete a helix, but now it is possible to select them and press the delete key, which is the convention for other objects such as strands. So we should remove the ability for pencil mode to delete a helix just by clicking it.

For the common use case that someone when creating helices accidentally clicks somewhere and wants to delete the helix they just created, Ctrl+Z is an equally easy way to do it.