6903893: Fix failed for CR 6834576: RemoveChild/ test fail with deadlock on rhel and sles

Reviewed-by: dcherepanov, art

jdk/src/solaris/classes/sun/awt/X11/XToolkit.java

@@ -1449,7 +1449,7 @@ public final class XToolkit extends UNIXToolkit implements Runnable {
      * (which is assigned to the virtual pointer) reports the maximum
      * capabilities of the mouse pointer (i.e. 32 physical buttons).
      */
-    private native synchronized int getNumberOfButtonsImpl();
+    private native int getNumberOfButtonsImpl();
 
     @Override
     public int getNumberOfButtons(){