FakeBkgTools: remove obsoleted conditional code aimed at standalone build

Context: it's possible to build the FakeBkgTools package (with its interfaces) on its own with no dependency to AnalysisBase / athena, in order to use it in downstream ntuple-reading frameworks that don't need the whole machinery. There's a small number of ifdefs here and there in the code that help with that (together with the emulated athena EDM types provided by https://gitlab.cern.ch/atlas-asg/AtlasFakeLeptonBackgroundTools).

A few of the recent changes in the athena repository broke the standalone build; in the process of fixing it, I decided to simplify the emulation of declareProperty/setProperty; they become a bit more fragile as a result, but not dramatically so.

Consequently, a few of the ifdef-protected bits are no longer necessary and can be removed to clean up a bit the source files in this repository.

This has no impact on the package when built as part of AnalysisBase or other athena projects.

cc @evarnes