Remove isSVGPath

This method is not needed since casting to RenderSVGShape is good enough, in SVGPathElement we only call methods on that class, nothing RenderSVGPath specific.

Review URL: https://codereview.chromium.org/411743002

git-svn-id: svn://svn.chromium.org/blink/trunk@178774 bbb929c8-8fbe-4397-9dbb-9b2b20218538