How remove fields from default price set
Even when you're not using a price set, modern versions of CiviCRM store the contribution amounts on a Contribution page as a Price Set internally.

CiviCRM doesn't let Price Fields be deleted when the Set has already been used to store data as potentially data loss could occur.

Therefore I think the best option is to disable them, or switch to using a real Price Set.

Also see Unable to delete Price Field from Price Set used for Contributions