[clang][dataflow] Remove deprecated `ValueModel::merge()` function. (#82602)

I'm not aware of any remaining overrides of this function.

While I'm here, change an outdated comment in DataflowAnalysis.h that
still
referred to `merge()`. I've made the comment more general, referring
simply to
`ValueModel`, as we shouldn't really be repeating the documentation of
that
class here anyway.