Don't chapter.reset() on change while drawing

(closes #5295)

The cause of this crash was:
- when cancelling a line, `drawWay.cancel()` needs to clean up old edits.
- it does this by looping `context.pop()` them off history until they are gone
- each `context.pop()` dispatches a 'change' event
- if we reset the chapter on one of these change events, it throws away the
   graph, and `drawWay.cancel()` can't finish what it's doing

modules/ui/intro/line.js

@@ -161,11 +161,6 @@ export function uiIntroLine(context, reveal) {
         }, 550);  // after easing..
 
         context.history().on('change.intro', function() {
-            var entity = _tulipRoadID && context.hasEntity(_tulipRoadID);
-            if (!entity) {
-                return chapter.restart();
-            }
-
             if (isLineConnected()) {
                 continueTo(continueLine);
             }