Drop isolated_as_missing and contig_id arguments in tskit.convert #413
Description
These should probably be considered part of tskit's responsibility to decide what the defaults of this should be, so we delegate repsonsiblitily there.
It's fine to just yank the parameters and notify users of the breakage in the changelog.
xref tskit-dev/tskit#3177
Note that we should do the same for contig_id, so that all tskit parameters are absorbed into the model_mapping object.