debugger: don't set the `repl.prompt` string

It wasn't doing anything, and actually due to
3ae0b17c76f693dd2e68a46f78c7dc7f595b33c6, it was causing
the readline `prompt()` function to be overwritten
which throws an error in the REPL shortly after.

See: https://github.com/joyent/node/commit/3ae0b17c76f693dd2e68a46f78c7dc7f595b33c6#commitcomment-5445121