Fix a11y on DividerWithText (#7482)

# Description of Changes
Nightlies are currently failing with an a11y issue on the
DividerWithText story. This fixes the a11y issue.

<img width="1440" height="1240" alt="image"
src="https://github.com/user-attachments/assets/c1623583-a68c-46d9-8cdb-453aca5ac20d"
/>

frontend/editor/src/proprietary/components/shared/dividerWithText/DividerWithText.css

@@ -16,11 +16,7 @@
 }
 
 .text-divider .text-divider__label {
-  /* A single channel triple cannot follow the colour scheme, and the old
-     fallback only reached 3.8:1 on the dark page. The muted text token carries
-     the readable shade for each scheme; the opacity hook is kept separate. */
   color: var(--c-text-muted);
-  opacity: var(--text-divider-opacity, 1);
   font-size: 0.75rem; /* 12px */
   white-space: nowrap;
 }
@@ -48,13 +44,6 @@
   );
 }
 
-.text-divider.force-light .text-divider__label {
-  color: rgb(
-    var(--text-divider-label-rgb-light, var(--gray-400)) /
-      var(--text-divider-opacity, 1)
-  );
-}
-
 .text-divider.subcategory.force-light .text-divider__rule {
   background-color: var(--tool-subcategory-rule-color-light);
 }