Remove unused file

PlatformMessagePortChannel.* isn't used. We use a version in the
dom/default/chromium directory.

R=eseidel@chromium.org, eseidel

Review URL: https://codereview.chromium.org/23672057

git-svn-id: svn://svn.chromium.org/blink/trunk@158229 bbb929c8-8fbe-4397-9dbb-9b2b20218538