Remove deprecated add_filter function

Hello @dfazzini,this function has been deprecated since a while but is still around and in fact used in the file itself where it i defined, see https://gitlab.cern.ch/lhcb/DaVinci/-/blob/master/Phys/DaVinci/python/DaVinci/algorithms.py#L51 and the rest of it. I think it is time to get rid of the function completely.

I noticed this from several WARNINGs running some DaVinciExample. So it seems we're filtering too many warnings, still, cf. the issue #79.