Remove now redundant ensure_dir
Remove now redundant ensure_dir

Since _build_one ensure the output dir is
present and does proper error logging,
we can remove ensure_dir from build(), further
simplifying that method.

pradyunsg/pip,sbidoul/pip,pypa/pip,pradyunsg/pip,xavfernandez/pip,xavfernandez/pip,pfmoore/pip,xavfernandez/pip,pypa/pip,pfmoore/pip,sbidoul/pip