The descriptor of an IO is never set to -2, remove dead code

Author: eregon

src/main/java/org/truffleruby/core/support/IONodes.java

@@ -396,11 +396,10 @@ public DynamicObject ensureOpen(DynamicObject file,
             if (fd == CLOSED_FD) {
                 errorProfile.enter();
                 throw new RaiseException(getContext(), coreExceptions().ioError("closed stream", this));
-            } else if (fd == -2) {
-                errorProfile.enter();
-                throw new RaiseException(getContext(), coreExceptions().ioError("shutdown stream", this));
+            } else {
+                assert fd >= 0;
+                return nil();
             }
-            return nil();
         }
 
     }