If you haven't installed Agile, then an administrator has probably added the field for some reason.
Talk to your other administrators first, to make sure you aren't going to break something they are working on.
Then, you need to think about how best to remove it from your projects - screens, field configurations or context.
I'm afraid that that's a bit of a self-contradiction
Fields will only "appear" if an administrator does something to add them. They can do it with plugins (you've seen Agile mentioned a few times already, but there may be others that add "sprint"), they can add fields, and they can uncover fields that were there before.
But it is something you have done to add it. The field will not just appear, it was enabled in some way by you.
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