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

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### WHY are these changes introduced?

Fixes https:/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:/Shopify/polaris/blob/main/documentation/Releasing.md#-snapshot-releases)
- [x] Tested on
[mobile](https:/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 = () => {