Fixed bug in BoostedJetMerger and made it a edm::stream::Producer

BoostedJetMerger had the wrong signature for beginJob and therefore
that function was never called (caught by clang). However, neither
beginJob nor endJob did anything so those methods were removed.
In addition, it was safe to change BoostedJetMerger to an
edm::stream::Producer and therefore become efficient when running
with multiple threads.