[ResourceItem] Consolidate conditional logic (#10172)

### WHY are these changes introduced?

Fixes #9961

### WHAT is this pull request doing?

* Consolidate se23 logic and styles for the `ResourceItem` 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-resourceitem--default&globals=polarisSummerEditions2023:true)
[This
PR](https://5d559397bae39100201eedc1-ganribjzxv.chromatic.com/?path=/story/all-components-resourceitem--default)

Author: sophschneider

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

@@ -23,10 +23,7 @@
   cursor: pointer;
 
   &:hover {
-    background-color: var(--p-color-bg-hover);
-    #{$se23} & {
-      background-color: var(--p-color-bg-subdued-hover);
-    }
+    background-color: var(--p-color-bg-subdued-hover);
 
     .Actions {
       // stylelint-disable-next-line selector-max-combinators -- show actions on hover
@@ -120,9 +117,7 @@
   .ListItem + & {
     border-top: var(--p-border-width-1) solid var(--p-color-border-subdued);
 
-    #{$se23} & {
-      @include item-separator;
-    }
+    @include item-separator;
   }
 
   &:not(.hasBulkActions):not(.selectMode) {