Unfortunately, there is not a way to remove the header bar unless you are in an app which is full-screen and chooses to hide that information. However, you can crop the image in the Photos app:
- Take Screenshot
- Open Photos app
- Open screenshot
- Press Edit in upper-right of screen
- Depending on rotation of screen, look for the Crop symbol
- Press Crop and drag the white boxed frame to match the image you'd like
- Press Done to finish cropping
Here is a Wikihow on how to do it, with screenshots.