fix: remove unused courseId variable in Textbooks component

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: farhan

src/textbooks/Textbooks.tsx

@@ -25,7 +25,7 @@ import messages from './messages';
 
 const Textbooks = () => {
   const intl = useIntl();
-  const { courseId, courseDetails } = useCourseAuthoringContext();
+  const { courseDetails } = useCourseAuthoringContext();
 
   const {
     textbooks,