prevent degenerate ways caused by deleting a corner of a triangle

[k-yle]

Closes #9329

@tyrasd had some alternative ideas in #9329 (comment), this PR implements option 1: you can no longer create a degenerate way by deleting the third-last node.

[k-yle]

There's one test failing:

iD/test/spec/actions/circularize.js

Line 281 in 4073609

iD.osmWay({id: '-', nodes: ['a', 'b', 'a']})

It's a bit of a weird test, because if you try to use the circularlise operation on a way that loops back on itself (A -> B -> A), nothing happens anyway. I think it's better to change update this test case.