Since Traders (as Sole Proprietors) don't incur Self-Employment tax on gains/losses, how do we remove the Self-Employment tax in Schedule C?

There should not be any income reported on your Schedule C for self-employment tax to be calculated.  The Schedule C should only be used to report your business expenses, not your income.

 

Your gains/losses should either be reported on Schedule D and Form 8949 or Form 4797 if you are covered under a Mark-to-Market election.  

 

Take a look at the following TurboTax help article for more information:

 

How do I report my trader activity with (or without) Section 475(f) election in TurboTax?