These ios updates download automatically if you have wifi on. If you dont have wifi on, they wont download. So once it downloads it sits and waits, it doesnt have to download anything anymore, it just annoys its customers by asking if they want to install it.
Do the following to get rid of it, go into:
storage and icloud
Find the entry for ios update, tap it then delete it.
I agree they should just give an option to NOT do this.