Can I use admob rewarded ads to remove banner ads?

Nothing is being said about this specific situation in Admob policies, not even in Admob's "Rewarded inventory policy" therefor I would consider it legal.

You may find more information on that topic on Admob's help site: Admob Help: Rewarded inventory policy

Since nothing like that is forbidden by the policy it should be allowed. First try to calculate your balance - one rewarded ad would probably bring you less money than a banner shown for 1 day. If it suits you don't worry and implement this feature, it seems totally legit.