Guessing as I've never tried it but this should in theory work: (get-media "test").ExtensionData.Delete()
The extension data is available for all objects. For example to get everything you can do to a CIvApp you can look at the following:
$civapp = get-civapp "Test"
$civapp.ExtensionData | gm
If you look at that you can see all the methods available:
$civapp.ExtensionData.PowerOn
$civapp.ExtensionData.Deploy
etc...
If you unsure of what parameters are required you can use the | fl on the method. Then look at the OverloadDefinitions for what is required:
eg: $civapp.ExtensionData.Deploy | fl
OverloadDefinitions : {System.Void Deploy(System.Nullable[bool] powerOn, System.Nullable[bool] forceCustomization, System.Nullable[int] deploymentLeaseSeconds)}
You will see what you need to pass it:
$civapp.ExtensionData.Deploy($false,$false,0)
Cheers!