[Frame] Only add rounded styles when topBar exists (#11925)

<!--
  ☝️How to write a good PR title:
- Prefix it with [ComponentName] (if applicable), for example: [Button]
  - Start with a verb, for example: Add, Delete, Improve, Fix…
  - Give as much context as necessary and as little as possible
  - Open it as a draft if it’s a work in progress
-->

### WHY are these changes introduced?

Fixes https://github.com/Shopify/polaris-internal/issues/1598

### WHAT is this pull request doing?

Only applies rounded frame styles when the `Frame` component has a
`topBar` prop

### How to 🎩

For `WithoutATopBar`, make sure
[story](https://5d559397bae39100201eedc1-agsvebvhin.chromatic.com/?path=/story/all-components-frame--without-a-top-bar)
is the same with feature flag on and off

https://admin.web.web-81d9.sophie-schneider.us.spin.dev/store/shop1

### 🎩 checklist

- [x] Tested a
[snapshot](https://github.com/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [x] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)

.changeset/mean-kangaroos-move.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+Updated Frame to only apply rounded Frame when passed a `topBar`

polaris-react/src/components/AppProvider/AppProvider.tsx

@@ -138,8 +138,6 @@ export class AppProvider extends Component<AppProviderProps, State> {
     const {i18n, linkComponent} = this.props;
 
     this.setRootAttributes();
-    /* Temporary for dynamicTopBarAndReframe feature. Remove when feature flag is removed. */
-    this.setBodyStyles();
 
     if (i18n === prevI18n && linkComponent === prevLinkComponent) {
       return;
@@ -152,20 +150,8 @@ export class AppProvider extends Component<AppProviderProps, State> {
   }
 
   setBodyStyles = () => {
-    const {features} = this.props;
-
     document.body.style.backgroundColor = 'var(--p-color-bg)';
     document.body.style.color = 'var(--p-color-text)';
-
-    /* Temporary for dynamicTopBarAndReframe feature.
-     * Remove when feature flag is removed and apply
-     * styles directly to body in the global stylesheet.
-     */
-    if (features?.dynamicTopBarAndReframe) {
-      document.body.style.overflow = 'hidden';
-    } else {
-      document.body.style.overflow = '';
-    }
   };
 
   setRootAttributes = () => {