Make HitContext fields final (#90784)

HitContext's SourceLookup field can no longer be swapped out, so we can make it
final.

server/src/main/java/org/elasticsearch/search/fetch/FetchSubPhase.java

@@ -25,7 +25,7 @@ class HitContext {
         private final SearchHit hit;
         private final LeafReaderContext readerContext;
         private final int docId;
-        private SourceLookup sourceLookup;
+        private final SourceLookup sourceLookup;
 
         public HitContext(SearchHit hit, LeafReaderContext context, int docId) {
             this.hit = hit;
@@ -65,10 +65,6 @@ public SourceLookup sourceLookup() {
             return sourceLookup;
         }
 
-        public void setSourceLookup(SourceLookup sourceLookup) {
-            this.sourceLookup = sourceLookup;
-        }
-
         public IndexReader topLevelReader() {
             return ReaderUtil.getTopLevelContext(readerContext).reader();
         }