Adjust CSS for expanded navigation section to remove unwanted space during collapse/expand (#8655)

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### WHY are these changes introduced?

Resolves #8653

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  Context about the problem that’s being addressed.
-->

### WHAT is this pull request doing?

There is a synchronization issue in which we have a sort of half-open
state for navigation sections. This moves the style from that half-open
state to the fully-open state.

The half-open state might still exist (because `Secondary` uses
`Collapsible` which wants to do animations) but AFAIK it's not causing
any unwanted behaviour.


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### How to 🎩

This can be tested if you check out my Navigation storybook improvements
in #8652 and cherry pick this commit over to that branch, or vice versa.
Try switching between sections, and record a high-speed video of it to
see the difference.

🖥 [Local development
instructions](https://github.com/Shopify/polaris/blob/main/README.md#local-development)
🗒 [General tophatting
guidelines](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md)
📄 [Changelog
guidelines](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#changelog)

### 🎩 checklist

- [ ] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [ ] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [x] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: stewx

.changeset/smart-pillows-buy.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': minor
+---
+
+Adjust CSS for expanded navigation section to remove unwanted space during collapse/expand

polaris-react/src/components/Navigation/Navigation.scss

@@ -381,14 +381,11 @@ $disabled-fade: 0.6;
   margin-left: 0;
   overflow-x: visible;
 
-  &.isExpanded {
-    margin-bottom: var(--p-space-2);
-  }
-
   .List {
     margin: 0;
     padding: 0;
     list-style: none;
+    margin-bottom: var(--p-space-2);
   }
 
   .Item {

polaris-react/src/components/Navigation/components/Item/Item.tsx

@@ -303,7 +303,6 @@ export function Item({
 
     const SecondaryNavigationClassName = classNames(
       styles.SecondaryNavigation,
-      showExpanded && styles.isExpanded,
       !icon && styles['SecondaryNavigation-noIcon'],
     );