Remove dart_model experimental code from language repo. See macros repo. (#4073)

Author: davidmorgan

.github/workflows/dart.yml

@@ -24,9 +24,6 @@ jobs:
       - name: dart pub get (working/macros/example)
         run: dart pub get
         working-directory: working/macros/example
-      - name: dart pub get (working/macros/dart_model)
-        run: testing/pub_get
-        working-directory: working/macros/dart_model
       - name: dart pub get (accepted/2.3/spread-collections/benchmarks)
         run: dart pub get
         working-directory: accepted/2.3/spread-collections/benchmarks