Remove PbMapMixin, Dart mixin support

[osa1]

This removes the last Dart mixin PbMapMixin. The mixin is not used
internally, and GitHub code search also doesn't find any uses other than in
forks of this repo (1, 2).

To keep the API surface small to make changes and migrations easier this PR
removes the mixin.

Because PbMapMixin was the only mixin left (after #880), this also removes
the Dart mixin support from the plugin.

cl/575127174

[osa1]

Note to self: clarify that this removes the built-in mixins. Users can still add arbitrary mixins to their message classes via the Dart options (https://github.com/google/protobuf.dart/blob/master/protoc_plugin/protos/dart_options.proto#L40).

Also mention this in the changelog.

[osa1]

It turns out this mixin has users. See cl/575127174 for details. Closing.