CosmosManyToManyJoinEntityTypeConvention should remove the properties on the join entity type corresponding to the old partition key

[ajcvickers]

See https://github.com/dotnet/efcore/pull/33536#pullrequestreview-2009733676

This was never implemented for non-hierarchical keys, and given the last of usage of join entities on Cosmos, it is quite low priority.

[AndriySvyryd]

Would be handled by #15898