LUDiagnoseOS._DiagnoseByOS: remove unused arg

The node_list argument to _DiagnoseByOS is not used, and is obsoleted by
the fact that the rlist argument already has the valid nodes as keys
(assuming RPC behaviour didn't change). Thus, we remove it and silence
the warning.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>