remove SVGElement::isContextElement

Original suggestion by rob.buis@samsung.com

All SVGElements are context element as |className| is a SVGAnimatedString. This flag is redundant.

BUG=None

Review URL: https://codereview.chromium.org/488843003

git-svn-id: svn://svn.chromium.org/blink/trunk@180614 bbb929c8-8fbe-4397-9dbb-9b2b20218538