templatekw: drop obsfatedata and related code
AFAIU, this function exists so that people could hack together a replacement
template for {obsfate} if they weren't satisfied with the default, but only on
older versions. It seems that 3d0f8918351b::d0f1e3d3ef4d was the series
introducing obsfate* functions that can show specific obsmarker properties, and
that was included in hg 4.4.
obsfatedata was already claimed to be removed in 35ffd7a4b339 and then more of
its usage was removed in 9882fed37ea9. There are no users in evolve and in core
hg, no tests and nothing else mentions obsfatedata.