fix($theme-default): Extending Layout.vue causes the Sidebar to disappear (close: #1045)

Author: ulivz

packages/@vuepress/theme-default/layouts/Layout.vue

@@ -86,7 +86,6 @@ export default {
     shouldShowSidebar () {
       const { frontmatter } = this.$page
       return (
-        !frontmatter.layout &&
         !frontmatter.home &&
         frontmatter.sidebar !== false &&
         this.sidebarItems.length