Remove os/src/app/cli_monitor

Nowadays, we use standard command-line tools like vim to edit init
configurations dynamically, which alleviates the need for a custom CLI.
The CLI-monitor component was too limited for use cases like Sculpt
anyway.

The patch also removes the ancient (and untested for long time)
terminal_mux.run script, which used to be the only remaining user of the
CLI monitor.

Issue #3512