SVG: Remove JSC specific workaround in SVGElement::removeEventListener

The workaround seems specific to JSC, and thus not needed in Blink.

BUG=None

Review URL: https://codereview.chromium.org/293983020

git-svn-id: svn://svn.chromium.org/blink/trunk@175778 bbb929c8-8fbe-4397-9dbb-9b2b20218538