Fix in Higgs HLT validation DQM for multithreading

**This PR is a fix in the Higgs HLT validation DQM to permit it to work multithreaded.**
The fix consists in filling the label of the efficiency summary when creating the histogram and not in the fly as before.

Two other small changes haves been done: 
- the summary and PU histograms are created directly from a ebooker method and not using a intermediate TH1F
- the summary of efficiency plots is now created using the DQM generic client and not  anymore using the HLTHiggsHarvesting class, which is removed.

*The content of the plots is not changed by this PR*