WebUSB: Serial "Enter REPL" button bar still visible when REPL activated

[microbit-carlos]

We might still want to provide a way to send the Ctrl+C command, but the current text makes it look as if perhaps the editor is not in "REPL" mode.

We shouldn't look into this one yet, as first we might need to reconsider the serial/REPL UX.

[microbit-carlos]

Ultimately this is by design, so closing this one as something we will keep.