MINOR: Remove unused local variable (#18410)

Reviewers: Luke Chen <showuon@gmail.com>

storage/src/main/java/org/apache/kafka/storage/internals/log/LocalLog.java

@@ -220,7 +220,6 @@ public class LocalLog {
      * @param newConfig the new configuration to be updated to
      */
     public void updateConfig(LogConfig newConfig) {
-        LogConfig oldConfig = config;
         config = newConfig;
     }