Well it would result in deflation because $1,500,000,000 disappears from the economy overnight. This means that the money supply shrinks and that kicks of a deflationary cycle.
There are two scenarios. In the first, the government simply prints the $1.5T. That's newly created money. $1.5T has been added to the economy overnight. It's been created, not destroyed.
The second scenario is where the government takes the money from somewhere else (maybe taxes). The money is removed from somewhere (someone's assets), but the exact same amount of debt is also removed. The net is zero - neither creation or destruction, just rearrangement.