AST#hashCode bugfix

Previous implementation always returned zero.
I can only assume that it wanted to cache it as well,
but I haven't implemented that to keep the changes light.

src/api/java/AST.java

@@ -72,12 +72,7 @@ public class AST extends Z3Object implements Comparable<AST>
     @Override
     public int hashCode()
     {
-        int r = 0;
+        return Native.getAstHash(getContext().nCtx(), getNativeObject());
-        try {
-            Native.getAstHash(getContext().nCtx(), getNativeObject());
-        }
-        catch (Z3Exception ex) {}
-        return r;
     }
 
     /**