on one server the update kb2920730 shows up in software center as failed, but this specific server has nothing installed related to SharePoint server

how can we get rid of this update?

the customer forced the machine to scan online and then it did not show that update in the list

how can we get rid of this update?

You can't - except you would remove it from the software update group. 

hmm, not what I was hoping for.

since removing the update from the SUG will prevent it to be installed on the few servers that will actually require the update...

That's also true. You cannot modify the detection logic. Try installing the update manually (on a test system) and see if it will install or if you will get the popup that the update is not applicable. 

I'm having exactly the same problem, it seems that this update shows up on Office Web Apps 2013 server, it is detected as required but when trying to install it it fails. I've already tried running manually, same deal, no go....

you should look at the logs why did the installation failed. check windowsupdate.log .did you try manual install and capture the error msg ?