Remove unused main_shell

[sjorsdonkers]

The Replace JS runtime PR removed the shell build target.

Shell was handled by the old jsruntime and has not been re-ported in browser.
It was useful at the beginning of the project, but we don't really use it now.
That said, it could be useful to have it, I had the idea of having an option --console in fetch
ie. the browser loads the web page and create a shell to interact with the js

main_shell.zig is right now useless, we should remove it, but maybe later we coud consider re-introduce it.