fix: remove non-existing :blur css pseudo selector (#3118)

There is no `:blur` CSS pseudo selector, meaning this generates invalid CSS.

Author: niknetniko

packages/@vuepress/theme-default/components/DropdownLink.vue

@@ -229,8 +229,6 @@ export default {
     &.open .nav-dropdown
       // override the inline style.
       display block !important
-    &.open:blur
-      display none
     .nav-dropdown
       display none
       // Avoid height shaked by clicking