[#11751] Deadline extensions in FeedbackSession entity have unnecessary constraint (#11752)

Author: samuelfangjw

src/main/java/teammates/storage/entity/FeedbackSession.java

@@ -76,9 +76,11 @@ public class FeedbackSession extends BaseEntity {
 
     private boolean isPublishedEmailEnabled;
 
+    @Unindex
     @Serialize
     private Map<String, Instant> studentDeadlines;
 
+    @Unindex
     @Serialize
     private Map<String, Instant> instructorDeadlines;