Change the default vertex fitter in FunctionalDiElectronMaker

The current vertex fitter in FunctionalDiElectronMaker is LoKi::VertexFitter, which is not aligned with the vertex fitter of the combiners and in addition it is not thread-safe. It has also caused problems in trigger lines for dielectron studies Moore#585, where efficiencies dropped by factors \mathcal{O}(4\%). It has been suggested that this might have to do with a bad performance of LoKi::VertexFitter when the two electrons are collinear, which would also affect studies at low-q^2 in b and c physics. We should align the vertex fitters in the combiners and the dielectron makers, but before we should validate that the changes in this MR do not have any significant impact on the trigger lines across the different working groups.