Remove runtime feature flag "WebAnimationsSVG"

Depends on chromium CL: https://codereview.chromium.org/530353002/

The flag is not used anymore.

BUG=None

Review URL: https://codereview.chromium.org/535463002

git-svn-id: svn://svn.chromium.org/blink/trunk@181353 bbb929c8-8fbe-4397-9dbb-9b2b20218538