Remove feature "requiring from the command line"

[Aaron Abbott]

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

In #227 (comment) 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.