[JENKINS-64931] no need to synchronize on Queue as queue is using a Lock and this under certain conditions can generate a deadlock. look at the dump attached to the issue JENKINS-64931 (#5305)

Signed-off-by: olivier lamy <olamy@apache.org>

core/src/main/java/hudson/model/queue/FutureImpl.java

@@ -71,16 +71,14 @@ public final class FutureImpl extends AsyncFutureImpl<Executable> implements Que
     @Override
     public boolean cancel(boolean mayInterruptIfRunning) {
         Queue q = Jenkins.get().getQueue();
-        synchronized (q) {
-            synchronized (this) {
-                if(!executors.isEmpty()) {
-                    if(mayInterruptIfRunning)
-                        for (Executor e : executors)
-                            e.interrupt();
-                    return mayInterruptIfRunning;
-                }
-                return q.cancel(task);
+        synchronized (this) {
+            if(!executors.isEmpty()) {
+                if(mayInterruptIfRunning)
+                    for (Executor e : executors)
+                        e.interrupt();
+                return mayInterruptIfRunning;
             }
+            return q.cancel(task);
         }
     }