Reimplement DynamicMapField to use UntypedMapBase directly.

[MurdoqZodoq]

Reimplement DynamicMapField to use UntypedMapBase directly.

It uses the same nodes that Map<> would use for the respectice types.
This makes the type compatible with visitation and removes some custom types
supporting DynamicMapField.

Remove most of the vtable, since this can now be done directly in MapFieldBase without derived type support.

Originally posted by @copybara-service in protocolbuffers/protobuf#20063

[jviau]

This issue does not appear to be applicable to functions host. It is a copy & paste of the PR description from an entirely unrelated repository.