refactor(reactivity): remove redundant variable declaration

Author: inottn

packages/reactivity/src/baseHandlers.ts

@@ -146,13 +146,12 @@ class MutableReactiveHandler extends BaseReactiveHandler {
   ): boolean {
     let oldValue = target[key]
     if (!this._isShallow) {
-      const isOldValueReadonly = isReadonly(oldValue)
       if (!isShallow(value) && !isReadonly(value)) {
         oldValue = toRaw(oldValue)
         value = toRaw(value)
       }
       if (!isArray(target) && isRef(oldValue) && !isRef(value)) {
-        if (isOldValueReadonly) {
+        if (isReadonly(oldValue)) {
           return false
         } else {
           oldValue.value = value