don't pass sitelists to merge jobsplitter

Merge jobs at the moment inherit the sitelists of their parent task (as they get passed in via the job splitting parameters). This is a) unneccessary and b) causes problems for opportunistic resources where the parent task runs at site A and merging should be run at site B.
Merge pull request #5362 from hufnagel/merge-no-sitelists

don't pass sitelists to merge jobsplitter