Can't unconditionally delete the node; it may be a member
of other ways.
I didn't preserve the behavior of dragging a midpoint to
an adjacent node being a no-op. In general we don't try to
eliminate compound operations whose net result is a no-op;
I don't think it's important to do so for this special case.
The degenerate case of connecting the endpoints of a two-vertex
line now results in a point. This is what naturally resulted
from the code, and seems ok.
Fixes#983.
