Remove feature "requiring from the command line"

See https://github.com/google/pprof-nodejs#requiring-from-the-command-line

In https://github.com/google/pprof-nodejs/pull/227#discussion_r1002072584 we discussed how moving to inspector API for time profiling makes the `stop()` method async. Since `stop()` is being run in a `process.on('exit')` handler, it can't be async and will fail in the new form.

This code is not used by the profiler agent so it should be safe to remove. If there are FRs to add it back, maybe `beforeExit` event would be a decent middleground, or we can add a bunch of signal handlers to run `stop()` before shutdown.