Remove unnecessary dependencies (#1696)

- Removes `plugin_platform_interface`
- Moves `yaml` to `dev_dependencies`

Author: brianquinlan

pkgs/objective_c/pubspec.yaml

@@ -21,8 +21,6 @@ dependencies:
   ffi: ^2.1.0
   flutter:
     sdk: flutter
-  plugin_platform_interface: ^2.0.2
-  yaml: ^3.1.0
 
 dev_dependencies:
   args: ^2.6.0
@@ -33,6 +31,7 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
   test: ^1.21.1
+  yaml: ^3.1.0
 
 dependency_overrides:
   ffigen: