SVG: Remove SVGAnimatedPropertyBase::startAnimation and its overrides which are already all NOOP

TESTS=svg
BUG=None

Review URL: https://codereview.chromium.org/304863002

git-svn-id: svn://svn.chromium.org/blink/trunk@175028 bbb929c8-8fbe-4397-9dbb-9b2b20218538