batman-adv: Don't return value in void function

batman-adv: Don't return value in void function

gw_node_delete is defined with "void" as return type, but still tries to
return a value. The called function gw_node_delete is also return as
void and thus doesn't provide a value for us.

Signed-off-by: Sven Eckelmann <sven@narfation.org>