remove_entries method

[stolarczyk]

I think it would be nice to have a remove_entries by key method since there is the add_entries.
Not sure what would be the best way to specify the level of the entry ot remove. Relevant in case there are multiple keys with the same name on different levels.

[vreuter]

I agree -- I think there should be symmetry. I prefer to remove add_entries from the API, though, first by deprecation then by actual removal. The only things it's doing differently than an ordinary update call are type checking the entries argument, handling the dispatch to __setitem__ to ensure that Mapping values become properly typed, and then returning then instance itself.
This could all be accomplished instead with an overridden update, which--besides solving the odd asymmetry--would be more familiar to users since update is an ordinary dict operation.