run-task: don't parse git clean's error output

We were sending stdout and stderr into the same stream and then
attempting to parse as if it were a list of paths.  That doesn't go
awesomely well when there's actual error output mixed in there.

Author: jcristau

src/taskgraph/run-task/run-task

@@ -604,7 +604,6 @@ def _clean_git_checkout(destination_path):
         # accurate.
         bufsize=0,
         stdout=subprocess.PIPE,
-        stderr=subprocess.STDOUT,
         stdin=sys.stdin.fileno(),
         cwd=destination_path,
         env=os.environ,