How do I remove messenger?

If it's an app that you've downloaded the App Store then press and hold any of the apps on your iPad’s homescreen and after a couple of seconds or so they should start to shake. Then press the 'x' in the left corner to delete the ones that you don't want, and when you've finished deleting press the home button so as to stop the shaking. If you don't get the 'x' on any of the apps that you've downloaded (you can't delete built-in apps in iOS 9 and below) check that Settings > General > Restrictions > Deleting Apps isn't set 'off'

If you are referring to the built-in Messages app then that is why you can't remove it, no built-in app can be removed in iOS 9 or below (the reason being that Apple didn't allow it). If you are referring to a 'messenger' app that you've downloaded from the App Store you can delete it using the instructions in my above reply.