Fix jenkins_tests.sh cleanup() trap

- Put everything in one function because each `trap … EXIT` overwrites the
  previous definition.
- Call `set +e` at the beginning so that we don't exit prematurely before
  all of the cleanup operations have been run.
- Unset the `FOG_RC` variable when we're done with it. Shouldn't make a
  different to the caller shell, but better safe than sorry and it's
  consistent with the other scripts like this.
- Remove the comment now that `cleanup()` is doing more than just deleting
  the temporary `FOG_RC` file.