Remove unused file from webkitpy

The FileSetFileHandle was only used by the ZipFileSet
which have already been removed from the webkitpy.

Review URL: https://codereview.chromium.org/641813008

git-svn-id: svn://svn.chromium.org/blink/trunk@184159 bbb929c8-8fbe-4397-9dbb-9b2b20218538