Fix HiDPI over-scrolling issue

If you visit https://rawgit.com/danvk/dygraphs/master/tests/demo.html on a "retina" device and shrink the window to match the width of the chart, you'll find that there's a mysterious horizontal scroll bar.

This was due to dygraph-canvas.js mysteriously re-setting the width of g.graphDiv. It's not clear why it would do this, since it's already been set in Dygraph.prototype.resizeElements_. It's also unclear why it would set the width and not the height. I'm guessing this is a relic -- the tests pass with the line removed.