Remove animatedPropertyCache from SVGAnimatedProperty, it is not needed anymore because of the new animated property system.

This is the first item on the removal TODO.

Review URL: https://codereview.chromium.org/180243018

git-svn-id: svn://svn.chromium.org/blink/trunk@168070 bbb929c8-8fbe-4397-9dbb-9b2b20218538