fix: go-playground-upload not working in playground snippet buffers

[jimeh]

This fixes the condition in the go-playground-upload function, so it runs go-play-buffer within go-playground snippet file buffers only. Previously it would run it in any buffer, except snippet buffers.

I also removed the cursor movement and insertion of the result from go-play-upload, as default behavior of go-play-buffer and go-play-region is to place the URL on the killring, and can be customized setting the go-play-browse-function variable.