fix edgehover bug when hovering arrow type edges

The size of edges did not look like what I expected when I hovered on the arrow edges, but it looked fine when I hover on the curve and curvedArrow edges. So I looked into the source code in `src/renderers/canvas/sigma.canvas.edgehovers.arrow.js`. I removed the code of `edge.hover` because it did not show in `../curve.js` or `../curvedArrow.js`. The size looks fine again. It seems that the value of `edge.hover` turns out always to be `false`.