Deletes all custom actions in the collection.

command: spo customaction clear --url|-u --scope|-s [scope] --confirm
url of the site (collection) to remove the user action from
scope of the custom action to remove. Allowed values Site|Web|All. Default All
requires connection to a SharePoint site

More information available at https://msdn.microsoft.com/en-us/library/office/dn531432.aspx#bk_UserCustomActionCollectionClear

@waldekmastykarz please align if needed.