HADOOP-14173. Remove unused AdlConfKeys#ADL_EVENTS_TRACKING_SOURCE. Contributed by John Zhuge.

Change-Id: I5dc6f885816b8834f718874542dfa373458b0333
(cherry picked from commit 5a40bafdaeec693e613aa02e79dbaaccfdab6f60)

hadoop-tools/hadoop-azure-datalake/src/main/java/org/apache/hadoop/fs/adl/AdlConfKeys.java

@@ -66,7 +66,6 @@ public final class AdlConfKeys {
   static final String ADL_HADOOP_CLIENT_NAME = "hadoop-azure-datalake-";
   static final String ADL_HADOOP_CLIENT_VERSION =
       "2.0.0-SNAPSHOT";
-  static final String ADL_EVENTS_TRACKING_SOURCE = "adl.events.tracking.source";
   static final String ADL_EVENTS_TRACKING_CLUSTERNAME =
       "adl.events.tracking.clustername";