chore: remove unused dependency

src/.vuepress/theme/enhanceApp.js

@@ -4,7 +4,6 @@ import { createStore } from '@theme/store/store'
 import VScrollLock from 'v-scroll-lock'
 import VueMq from 'vue-mq'
 import { VLazyImagePlugin } from 'v-lazy-image'
-import VueObserveVisibility from 'vue-observe-visibility'
 import VueSocialSharing from 'vue-social-sharing'
 import Transition from '@theme/components/directives/Transition.js'
 
@@ -39,7 +38,6 @@ export default ({ Vue, router, siteData, isServer }) => {
   Vue.use(VScrollLock)
   Vue.use(VueMq, { breakpoints })
   Vue.use(VLazyImagePlugin)
-  Vue.use(VueObserveVisibility)
   Vue.use(VueSocialSharing)
 
   Vue.directive(Transition.name, Transition.directive)