[ABFS] Reverting Back Support of setXAttr() and getXAttr() on root path

A while back changes were made to support HDFS.setXAttr() and HDFS.getXAttr() on root path for ABFS Driver.
For these, filesystem level APIs were introduced and used to set/get metadata of container.
Refer to Jira: HADOOP-18869 ABFS: Fixing Behavior of a File System APIs on root path - ASF JIRA (apache.org)

Ideally, same set of APIs should be used, and root should be treated as a path like any other path.
This change is to avoid calling container APIs for these HDFS calls.

As a result of this these APIs will fail on root path (as earlier) because service does not support get/set of user properties on root path.
This change will also update the documentation to reflect that these operations are not supported on root path.