Remove languageId from markdown cells

[christian-bromann]

It seems that for markdown sections we return a languageId property that is always empty. I guess we can remove this?

[adambabik]

I was thinking about hiding it when it's empty but if I read TS correctly, NotebookCellData requires presence of languageId always.

[christian-bromann]

That is true, however it should not be a concern for the runme CLI as it should provide an ideal interface for what it is doing and not be too dependent on VS Code. In fact we don't even use/access the property. So there is not really a value to be 1:1 compliant with the VS Code API.