Remove MyGet mentions from samples

Although MyGet information has been removed from our root `README.md`, the related information in all samples still need to be removed. 

![image](https://user-images.githubusercontent.com/14900841/97337863-053f3700-183e-11eb-8f87-ec6a1513d67b.png)

related to: https://github.com/microsoft/BotFramework-WebChat/issues/3355