Try this code in a cube view extender rule
Dim uiItem As CVExtenderReportUIItem = args.Report.CurrentUIItem
If uiItem.UIItemType = XFReportUIItemType.PageHeaderPictureBoxLogo Then
uiItem.Visible = False
(not tested my self)
An alternate solution could be to create a new Dashboard with a Report Component. Inside the Report Component is the Report Designer tab. The Report Designer is a reporting design interface where you can customize the look and feel of a specific Dashboard. Here you could delete the logo as a Picture Box Image and save as a new Dashboard without the logo.