Jan. 26, 2021 update: Facebook has just announced a new experience for Business Pages that should be available in the upcoming months. If you are not able to remove the tab from your Page using the procedure below, please be patient. The new page experience will fix the issue.
Facebook is buggy and incomplete when it comes to managing Page tabs and the only workaround to remove a tab is to use the old Facebook interface. To switch to the old interface, open your Facebook page in a browser and replace the subdomain in the address bar from www to business. The new URL should look something like https://business.facebook.com/mypage where "mypage" is your page username.