utils: Remove process.pipe_async

pipe_async was broken by design (e.g. it couldn't handle when there was
no output from the process) and it was unused, so removing it.

Change-Id: Ica3f0af0c663e9a1bc52b33bb70091e595b3df5f
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>