remove a needless if. (#3034)

Since always remove the old class.

src/directives/internal/transition.js

@@ -13,9 +13,7 @@ export default {
     id = id || 'v'
     oldId = oldId || 'v'
     el.__v_trans = new Transition(el, id, hooks, this.vm)
-    if (oldId) {
-      removeClass(el, oldId + '-transition')
-    }
+    removeClass(el, oldId + '-transition')
     addClass(el, id + '-transition')
   }
 }