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

@@ -599,13 +599,8 @@ def _clean_git_checkout(destination_path):
     print_line(b"vcs", b"executing %r\n" % args)
     p = subprocess.Popen(
         args,
-        # Disable buffering because we want to receive output
-        # as it is generated so timestamps in logs are
-        # accurate.
-        bufsize=0,
         stdout=subprocess.PIPE,
-        stderr=subprocess.STDOUT,
-        stdin=sys.stdin.fileno(),
+        stdin=subprocess.DEVNULL,
         cwd=destination_path,
         env=os.environ,
     )