Remove superfluous AnnotationAttributes#putIfAbsent implementation

Since Java 8, putIfAbsent() is a standard method in java.util.Map. We
therefore no longer need the custom implementation that overrides the
standard implementation in HashMap.

spring-core/src/main/java/org/springframework/core/annotation/AnnotationAttributes.java

@@ -369,26 +369,6 @@ public class AnnotationAttributes extends LinkedHashMap<String, Object> {
 		}
 	}
 
-	/**
-	 * Store the supplied {@code value} in this map under the specified
-	 * {@code key}, unless a value is already stored under the key.
-	 * @param key the key under which to store the value
-	 * @param value the value to store
-	 * @return the current value stored in this map, or {@code null} if no
-	 * value was previously stored in this map
-	 * @see #get
-	 * @see #put
-	 * @since 4.2
-	 */
-	@Override
-	public Object putIfAbsent(String key, Object value) {
-		Object obj = get(key);
-		if (obj == null) {
-			obj = put(key, value);
-		}
-		return obj;
-	}
-
 	@Override
 	public String toString() {
 		Iterator<Map.Entry<String, Object>> entries = entrySet().iterator();