To remove associated revenue from an opportunity, follow the steps below.

1) Within the Associated revenue tab of the Opportunity Page, click the value in the Amount field (it is a hyperlink). This will take you to the Revenue Transaction Page.
2) Under the Application details section, click Go to revenue which will take you to the Revenue Application Page.
3) Click Edit Details > Opportunity drop-down > Clear the value > Save.
4) Go back to the Opportunity Page and notice that the associated revenue has been removed.

Also, there is a suggestion on the Ideas Portal to add a Remove task on the Associated revenue tab to streamline the process: https://bbcrm.ideas.aha.io/ideas/CRM-I-1067.