remove workspace from workspace collection

Oddly enough, I did not see that functionality when the first released the API. In fact, when I gave my talk on Power BI Embedded about a month ago, I called out the "minimal viable product" nature of Power BI development and used this as an example. 

However, it seems that they have added that capability:

https://msdn.microsoft.com/library/mt712306.aspx

https://msdn.microsoft.com/en-us/library/mt712297.aspx