Remove unused field in one of junitlauncher task supporting class

src/main/org/apache/tools/ant/taskdefs/optional/junitlauncher/StandaloneLauncher.java

@@ -202,7 +202,6 @@ public class StandaloneLauncher {
 
         private boolean printSummary;
         private boolean haltOnFailure;
-        private TestExecutionContext testExecutionContext;
         private List<TestDefinition> tests = new ArrayList<>();
         private List<ListenerDefinition> listeners = new ArrayList<>();