remove unnecessary comment (#529)

Author: Carroll Chiou

src/main/java/org/jenkinsci/plugins/workflow/cps/CpsFlowExecution.java

@@ -415,12 +415,7 @@ private Timing(TimingKind kind) {
             start = System.nanoTime();
         }
 
-        @SuppressFBWarnings("RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE")
         @Override public void close() {
-            // it is possible for timings to be null if an old build (< v2686.v7c37e0578401) is being loaded from a saved state
-            if (timings == null) {
-                timings = new ConcurrentHashMap<>();
-            }
             timings.merge(kind.name(), System.nanoTime() - start, Long::sum);
         }
     }