Remove shape-padding

This patch removes shape-padding support, since it can be used only with shape-inside. Shape-inside support has been removed in r170260.

BUG=356778

Review URL: https://codereview.chromium.org/216793009

git-svn-id: svn://svn.chromium.org/blink/trunk@170590 bbb929c8-8fbe-4397-9dbb-9b2b20218538