fix: remove hasOwnProperty workaround

vue-test-utils-next now uses jest v27.1+, which includes a fix for hasOwnPropery (see jestjs/jest#11721).
This allows to remove the dirty workaround we had in our codebase, but users will have to update to Jest v27.1+.

src/mount.ts

@@ -464,10 +464,6 @@ export function mount(
   // if not, use the return value from app.mount.
   const appRef = vm.$refs[MOUNT_COMPONENT_REF] as ComponentPublicInstance
   const $vm = Reflect.ownKeys(appRef).length ? appRef : vm
-  // we add `hasOwnProperty` so jest can spy on the proxied vm without throwing
-  $vm.hasOwnProperty = (property) => {
-    return Reflect.has($vm, property)
-  }
   console.warn = warnSave
   return createWrapper(app, $vm, setProps)
 }