Revert "Emit a 'stalled' event." (it's misleading)

Created by: bradvogel

Reverts OptimalBits/bull#254

@manast I realized that this event is actually misleading. New jobs are being processed through processStalledJobs even though they haven't been attempted before.

For example:

1. Worker 1: creates job
2. Worker 2: picks up job in getNextJob() and moves it from 'wait' to 'active'
3. Worker 3: runs processStalledJobs immediately after and gets the job lock before Worker 2 can get it.

I'm testing this in production with 1M+ jobs/day with 20 workers, and seeing more jobs get picked up by processStalledJobs than are picked up by takeLockAndProcessJob.

So if that's the case, this event is misleading.