remove no-longer neeeded check from lxcapi_attach_run_waitl

commit f4364484454c754ed905bfb85bd2727a0a5d0475 introduced attach
functionality to unprivileged containers so remove this check like
the rest of the attach functions.

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>