[CQ] remove FlutterCreateAdditionalSettingsField deadcode (#8207)

Following up on a TODO from Steve to remove an unused method and
callers.

Interestingly, `FlutterModuleGroup` seems entirely unused. It's meant
to:

```
// Define a group of Flutter project types for use in Android Studio 4.2 and later.
```

Steve warns:

```
// TODO (messick) DO NOT delete this during post-4.2 clean-up.
```

but maybe we can? Especially if noone has plans to re-integrate.

@jwren : do you have any context on this?


---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

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Author: pq

flutter-idea/src/io/flutter/module/settings/FlutterCreateAdditionalSettingsFields.java

@@ -142,10 +142,6 @@ public Insets getBorderInsets(Component c, Insets insets) {
     });
   }
 
-  public void updateProjectType(FlutterProjectType projectType) {
-    // TODO(messick) Remove this method and its caller, which is in the flutter-studio module.
-  }
-
   public FlutterCreateAdditionalSettings getAdditionalSettings() {
     return new FlutterCreateAdditionalSettings.Builder()
       .setDescription(!descriptionField.getText().trim().isEmpty() ? descriptionField.getText().trim() : null)

flutter-studio/src/io/flutter/project/FlutterModuleGroup.java

@@ -26,17 +26,15 @@ public String getBuilderId() {
   public ModuleWizardStep getCustomOptionsStep(final WizardContext context, final Disposable parentDisposable) {
     // This runs each time the project type selection changes.
     FlutterModuleWizardStep step = (FlutterModuleWizardStep)super.getCustomOptionsStep(context, parentDisposable);
-    assert step!= null;
+    assert step != null;
     getSettingsField().linkHelpForm(step.getHelpForm());
-    setProjectTypeInSettings();
     return step;
   }
 
   @Override
   public ModuleWizardStep modifySettingsStep(@NotNull SettingsStep settingsStep) {
     // This runs when the Next button takes the wizard to the second page.
     ModuleWizardStep wizard = super.modifySettingsStep(settingsStep);
-    setProjectTypeInSettings(); // TODO (messick) Remove this if possible (needs testing).
     return wizard;
   }
 
@@ -46,10 +44,6 @@ public String getParentGroup() {
     return "Flutter";
   }
 
-  protected void setProjectTypeInSettings() {
-    getSettingsField().updateProjectType(getFlutterProjectType());
-  }
-
   public static class App extends FlutterModuleGroup {
 
     @NotNull