How do I remove the WPMU Dashboard message from client's site?

I'm trying to remove this message, but I have gone so far as to actually delete all WPMU plugins including the Dashboard and I still get this red ad to try the Dashboard on every WordPress admin page.

This is for a client's site and they don't want or need any of the previously tested WPMU plugins.

I don't think I should have to add the plugin back to the site in order to disable the message. That suggests that even if I delete the plugin, there is still WPMU code left in my DB.

Unless I've missed something, when I delete a plugin, shouldn't it remove all code and custom tables from my database? What's up with that?

How do I remove all the WPMUDev code from my client's site?