Remove unused argument 'defer' in function 'job_start'.

* With the changes in commit
  "events: Refactor how event deferral is handled"
  (2e4ea29d2c7b62eb8baf1c41cd43433e085dda0) the function argument
  'defer' of 'job_start' and member variable 'defer' of 'struct job'
  can be removed.
* Update/Fix the documentation for function 'job_start'.