Remove unused code from WebCore/platform/FileSystem.{h,cpp}

Also removes unused code in WebCore/platform/FileStream.{h,cpp}.

This leaves WebCore/platform/chromium/FileSystem* files as is for now.

BUG=none

Review URL: https://codereview.chromium.org/14137007

git-svn-id: svn://svn.chromium.org/blink/trunk@148324 bbb929c8-8fbe-4397-9dbb-9b2b20218538