You can feel free to leave it out.  The Lipton onion soup mix contains roughly 6,000 mg of sodium for one package.  If you add this to already spiced up soup that may also have boullion cubes in it then you are probably taking in way more sodium than you need.  This can only lead to fluid retention which you don't need when trying to lose weight.  It can be discouraging, if nothing else.  Plus, you need to lose the fluid somewhere.  Our bodies are more than 70 % water. 

All that being said, people outside the US who cannot find Lipton often can find an equivalent brand called Knorr instead.