Reduce code duplication in SystemProperties

This will replace a rarely seen log message saying "not found" with
one that says "default: ... => null", but that seems OK.

core/src/main/java/jenkins/util/SystemProperties.java

@@ -183,26 +183,7 @@ public class SystemProperties {
      */
     @CheckForNull
     public static String getString(String key) {
-        String value = System.getProperty(key); // keep passing on any exceptions
-        if (value != null) {
-            if (LOGGER.isLoggable(Level.CONFIG)) {
-                LOGGER.log(Level.CONFIG, "Property (system): {0} => {1}", new Object[] {key, value});
-            }
-            return value;
-        }
-        
-        value = handler.getString(key);
-        if (value != null) {
-            if (LOGGER.isLoggable(Level.CONFIG)) {
-                LOGGER.log(Level.CONFIG, "Property (context): {0} => {1}", new Object[]{key, value});
-            }
-            return value;
-        }
-        
-        if (LOGGER.isLoggable(Level.CONFIG)) {
-            LOGGER.log(Level.CONFIG, "Property (not found): {0}", key);
-        }
-        return null;
+        return getString(key, null);
     }
 
     /**