HADOOP-18752. Change fs.s3a.directory.marker.retention to "keep" -everything but the switch

update testing.md to remove statement that default is keep

Change-Id: Ic28ad7d9fe17566ee0603b3f6fc41f27df754222

Author: steveloughran

hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/testing.md

@@ -346,8 +346,6 @@ Example: test with `markers=keep`
 mvn verify -Dparallel-tests -DtestsThreadCount=4 -Dmarkers=keep
 ```
 
-This is the default and does not need to be explicitly set.
-
 Example: test with `markers=delete`
 
 ```