jenkins service: remove unneeded (and brittle) part of postStart

The current postStart code holds Jenkins off the "started" state until
Jenkins becomes idle. But it should be enough to wait until Jenkins
start handling HTTP requests to consider it "started".

More reasons why the current approach is bad and we should remove it,
from @coreyoconnor in
https://github.com/NixOS/nixpkgs/issues/14991#issuecomment-216572571:

  1. Repeatedly curling for a specific human-readable string to
  determine "Active" is fragile. For instance, what happens when jenkins
  is localized?

  2. The time jenkins takes to initializes is variable. This (at least
  used to) depend on the number of jobs and any plugin upgrades requested.

  3. Jenkins can be requested to restart from the UI. Which will not
  affect the status of the service. This means that the service being
  "active" does not imply jenkins is initialized. Downstream services
  cannot assume jenkins is initialized if the service is active. Might
  as well accept that and remove the initialized test from service
  startup.

Fixes #14991.