Remove `Markdown` in the default language drop down

[shibbas]

Is your feature request related to a problem? Please describe. When creating a new polyglot notebook, you can select the default language from a drop down. Markdown is also one of the languages, however it's not the language of the root kernel.

Describe the solution you'd like Unsure, if this drop down is supposed to represent default cells created in which language or the set the default language of the root kernel. IF latter, then it should probably not allow Markdown as an option.