Xcode 7+
Click your project name in the Project Navigator. Then select your Tests target. Click the minus (-) button at the bottom to delete it. (Select everywhere in red in the image below.)

As mentioned in a comment earlier, it is a good idea to do Unit Testing (and UI testing) in your projects. So, if you want to add them back in then just click the plus (+) button.
See also
- How to do a Unit Test in Xcode
- Xcode UI Test example