Don't use btaggingLink in FlavorTagDiscriminants

Keeping track of data dependencies is easier if we retrieve containers directly rather than getting them via a link from another container. This merge makes some changes in FlavorTagDiscriminants to avoid accessing btagging via the link we store in jets.

Part of this is changing the tool interface for b-tag decorations so that we pass around BTagging objects rather than Jet objects. I'm also deleting a few wrapper algorithms in favor of BTagDecoratorAlg, which is slightly more generic.

This shouldn't change anything in the output files: I compared a hundred or so events and saw no differences in any branches.