_dbus_credentials_to_string_append: Remove useless join = FALSE

_dbus_credentials_to_string_append: Remove useless join = FALSE

It can't actually matter in practice, because we never know the
Unix uid and Unix groups but not pid, and we never have a Windows SID
and also a Linux security label; but resetting join to FALSE can only
ever result in us outputting something like "foo=123bar=456" instead
of the intended form with a space in the middle.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=103737