oeqa/concurrencytest: Remove invalid buffering option

Fix warnings from oe-selftest -j:

/usr/lib/python3.10/os.py:1030: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
  return io.open(fd, mode, buffering, encoding, *args, **kwargs)

Remove the option since it clearly doesn't do much.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6b872ee7)
Signed-off-by: Steve Sakoman <steve@sakoman.com>