[RFC] Remove unused argument 'defer' in function 'job_start'.

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`.

ping @tarruda to verify nothing else is planned for the `defer` member variable of `struct job`.