fix(svelte-query): simplify creatMutation sub/unsub

Author: hmnd

packages/svelte-query/src/createMutation.svelte.ts

@@ -1,5 +1,3 @@
-import { onDestroy } from 'svelte'
-
 import { MutationObserver, noop, notifyManager } from '@tanstack/query-core'
 import { useQueryClient } from './useQueryClient.js'
 import { watchChanges } from './utils.svelte.js'
@@ -57,22 +55,13 @@ export function createMutation<
 
   let result = $derived(observer.getCurrentResult())
 
-  const subscribe = (
-    observer: MutationObserver<TData, TError, TVariables, TContext>,
-  ) =>
-    observer.subscribe((val) => {
+  $effect.pre(() => {
+    const unsubscribe = observer.subscribe((val) => {
       notifyManager.batchCalls(() => {
         Object.assign(result, val)
       })()
     })
-  let unsubscribe = $state(subscribe(observer))
-
-  $effect.pre(() => {
-    unsubscribe = subscribe(observer)
-  })
-
-  onDestroy(() => {
-    unsubscribe()
+    return unsubscribe
   })
 
   const resultProxy = $derived(