Remove runkit tool from the hello world page

[bjohansebas]

It could be interesting if the page allowed you to run an Express server and test it directly, but we're using Runkit and it seems to always use Node 10

Since Express 5 requires Node.js 18, I think it's time to remove this tool as it can cause confusion in some cases due to the Node.js version (@expressjs/express-tc)

[crandmck]

I believe you can pick the Node version from the dropdown, incl v 18:

Instead of removing it altogether, perhaps just say to set the required min version for the desired Express version...?

[crandmck]

OK based on #1632 it seems it's not working at all, so yes let's get rid of it.