refactor($core): remove 'custom' prop from 'Content' component

Author: ulivz

packages/@vuepress/core/lib/app/components/Content.js

@@ -5,10 +5,6 @@ export default {
   functional: true,
 
   props: {
-    custom: {
-      type: Boolean,
-      default: true
-    },
     pageKey: String,
     slot: String
   },
@@ -24,7 +20,7 @@ export default {
       }
 
       return h(pageKey, {
-        class: [props.custom ? 'custom' : '', data.class, data.staticClass],
+        class: [data.class, data.staticClass],
         style: data.style,
         slot: props.slot || 'default'
       })

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

@@ -38,7 +38,7 @@
       </div>
     </div>
 
-    <Content custom/>
+    <Content class="custom"/>
 
     <div
       class="footer"

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

@@ -2,7 +2,7 @@
   <div class="page">
     <slot name="top"/>
 
-    <Content :custom="false"/>
+    <Content/>
 
     <div class="page-edit">
       <div