HADOOP-18416. fix ITestS3AIOStatisticsContext test failure (#4931)

Uncomment the S3ATestUtils-side part of the original patch.

hadoop-tools/hadoop-aws/src/test/java/org/apache/hadoop/fs/s3a/S3ATestUtils.java

@@ -1508,7 +1508,6 @@ public final class S3ATestUtils {
    * @param conf Configuration to remove the prefetch property from.
    */
   public static void disablePrefetching(Configuration conf) {
-    // TODO: Reinstate once HADOOP-18028 is merged.
-    // removeBaseAndBucketOverrides(conf, PREFETCH_ENABLED_KEY);
+    removeBaseAndBucketOverrides(conf, PREFETCH_ENABLED_KEY);
   }
 }