docs: remove duplicated example for watchEffect

Author: AloisSeckar

src/api/reactivity-core.md

@@ -278,19 +278,6 @@ Runs a function immediately while reactively tracking its dependencies and re-ru
   // -> logs 1
   ```
 
-  Side effect cleanup:
-
-  ```js
-  watchEffect(async (onCleanup) => {
-    const { response, cancel } = doAsyncWork(id.value)
-    // `cancel` will be called if `id` changes
-    // so that previous pending request will be cancelled
-    // if not yet completed
-    onCleanup(cancel)
-    data.value = await response
-  })
-  ```
-
   Stopping the watcher:
 
   ```js