Remove AbstractJob#setParentJobStatus

AbstractJob#setParentJobStatus was added as a quick fix for XCOMMONS-2756 as it was blocking XWIKI-21120 which was an important regression we needed to have fixed before releasing 14.10.6 and 15.5.2.

Now tmortagne suggested in XCOMMONS-2756 to actually revert XCOMMONS-1688 which was the cause of XWIKI-21120 instead of adding this new API. This is more dangerous than adding that new API, that's why we didn't go there before. But we could now decide to remove the unstable API and to perform the revert of XCOMMONS-1688.