[FullscreenBar] Consolidate conditional logic (#10173)

### WHY are these changes introduced?

Fixes #9937

### WHAT is this pull request doing?

* Consolidate se23 logic and styles for the `FullscreenBar` Component

### How to 🎩
* Compare production and this PR's chromatic storybook to make sure
styles are the same


[Production](https://storybook.polaris.shopify.com/?path=/story/all-components-fullscreenbar--all&globals=polarisSummerEditions2023:true)
[This
PR](https://5d559397bae39100201eedc1-duqojrtsgy.chromatic.com/?path=/story/all-components-fullscreenbar--all)

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

@@ -4,37 +4,23 @@
   position: relative;
   display: flex;
   height: $top-bar-height;
-  box-shadow: var(--p-shadow-sm);
+  box-shadow: var(--p-shadow-xs);
   background-color: var(--p-color-bg);
-
-  #{$se23} & {
-    box-shadow: var(--p-shadow-xs);
-  }
 }
 
 .BackAction {
   display: flex;
   flex: 0 1 auto;
   align-items: center;
-  padding-left: var(--p-space-3);
-  padding-right: var(--p-space-3);
+  padding-left: var(--p-space-4);
+  padding-right: var(--p-space-4);
   border-width: 0;
   border-right: var(--p-border-width-1) solid var(--p-color-border-subdued);
   background-color: var(--p-color-bg);
   font-weight: var(--p-font-weight-medium);
   cursor: pointer;
-
-  #{$se23} & {
-    padding-left: var(--p-space-4);
-    padding-right: var(--p-space-4);
-  }
 }
 
 .BackAction :first-child {
-  padding-right: var(--p-space-05);
-
-  #{$se23} & {
-    margin-right: var(--p-space-1_5-experimental);
-    padding-right: 0;
-  }
+  margin-right: var(--p-space-1_5-experimental);
 }