Remove some unused functions in RenderStyle

Trivial patch, found while investigating another issue. This patch
removes resetMargin and resetBorderRadius.

Review URL: https://codereview.chromium.org/337043002

git-svn-id: svn://svn.chromium.org/blink/trunk@176258 bbb929c8-8fbe-4397-9dbb-9b2b20218538