Remove util.subproc.Pipe, as we no longer use it in BE.

Remove util.subproc.Pipe, as we no longer use it in BE.

This was at one point used by `update_copyright.py`.  Now that that is
an external package (and doesn't use `Pipe` anymore either), we can
safely remove this code.

As a side benifit, the Pipe doctests will no longer be there to fail
on OS X and other systems sufficiently different from my development
box.