Removing dead code related to webkit-prifixed canvas2d line dash feature

This API never shipped in Chrome (bindings were only implemented for JSC and
not V8) so this code has been dead since the blink fork.

BUG=341566
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/143573007

git-svn-id: svn://svn.chromium.org/blink/trunk@166656 bbb929c8-8fbe-4397-9dbb-9b2b20218538