Interface cleanup for WebGestureCurveTarget

This is the final patch in a three-part series towards early termination of
flings when scrolling is impossible, cleaning up now unused portions of the
WebGestureCurveTarget interface.

Blink patch (#1)
https://codereview.chromium.org/139493003/

Chromium patch (#2):
https://codereview.chromium.org/136173004/

Review URL: https://codereview.chromium.org/217893004

git-svn-id: svn://svn.chromium.org/blink/trunk@170887 bbb929c8-8fbe-4397-9dbb-9b2b20218538