[Thumbnail] Consolidate conditional logic (#10171)

### WHY are these changes introduced?

Fixes #9973

### WHAT is this pull request doing?

* Consolidate se23 logic and styles for the `Thumbnail` 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-thumbnail--all&globals=polarisSummerEditions2023:true)
[This PR]()

Author: sophschneider

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

@@ -14,41 +14,31 @@
   // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
   min-width: var(--pc-thumbnail-extra-small-size);
   max-width: 100%;
-  border-radius: var(--p-border-radius-1);
-  border: var(--p-border-width-1) solid var(--p-color-border-subdued);
+  border-radius: var(--p-border-radius-2);
 
-  #{$se23} & {
+  &::after {
+    content: '';
+    top: 0;
+    left: 0;
+    right: 0;
+    bottom: 0;
+    position: absolute;
     border-radius: var(--p-border-radius-2);
-    border: none;
-
-    &::after {
-      content: '';
-      top: 0;
-      left: 0;
-      right: 0;
-      bottom: 0;
-      position: absolute;
-      border-radius: var(--p-border-radius-2);
-      box-shadow: var(--p-shadow-border-inset-experimental);
-    }
-
-    &.sizeExtraSmall,
-    &.sizeExtraSmall::after {
-      border-radius: var(--p-border-radius-1_5-experimental);
-    }
+    box-shadow: var(--p-shadow-border-inset-experimental);
+    display: block;
+    // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
+    padding-bottom: 100%;
+  }
 
-    &::before {
-      content: '';
-      display: block;
-      // stylelint-disable-next-line -- SE23 styling
-      padding-bottom: 100%;
-    }
+  &.sizeExtraSmall,
+  &.sizeExtraSmall::after {
+    border-radius: var(--p-border-radius-1_5-experimental);
   }
 
-  &::after {
+  &::before {
     content: '';
     display: block;
-    // stylelint-disable-next-line -- generated by polaris-migrator DO NOT COPY
+    // stylelint-disable-next-line -- SE23 styling
     padding-bottom: 100%;
   }
 }