Pex_policy support removed from obs_base and afw.image.Filter

I’ve just merged DM-15767 which implements RFC-525, which removes support for using pex_policy .paf files to configure Mappers and Filters.

Both of these have future-proof alternatives that were already more popular (YAML for mapper configuration and the lsst.afw.image.defineFilter free function), but there may be some code (in rarely-updated obs packages in particular) broken by this change. I’ve tried to fix everything I could find (and Jenkins is of course green), but I’m not positive I found everything.