HBASE-26105 Rectify the expired TODO comment in CombinedBC. (#3509)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CombinedBlockCache.java

@@ -74,8 +74,6 @@ public class CombinedBlockCache implements ResizableBlockCache, HeapSize {
   @Override
   public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching,
       boolean repeat, boolean updateCacheMetrics) {
-    // TODO: is there a hole here, or just awkwardness since in the lruCache getBlock
-    // we end up calling l2Cache.getBlock.
     // We are not in a position to exactly look at LRU cache or BC as BlockType may not be getting
     // passed always.
     boolean existInL1 = l1Cache.containsBlock(cacheKey);