Aligning guidelines: remove line after render

I know this is not an official fabric.js lib, so feel free to remove this PR.

After using aligning guidelines with transparent lines (opacity=0.5), it seems that vertical et horizontal lines are only removed after a mouse up (while new lines a created after each object:moving).

So as long as you don't "mouse up", new lines are created and "superimposed" (which is visible only with semi-transparent lines).

Fix this bug: remove lines after each render.