Remove unneeded logging for nested sidebar groups

This arbitrarily logged out when nesting was greater than 3.

```
07:58:13.598 [vuepress] detected a too deep nested sidebar group. 2 2.258cb917.js:1:3364
```

vuejs#2190

packages/@vuepress/theme-default/util/index.js

@@ -222,11 +222,6 @@ function resolveItem (item, pages, base, groupDepth = 1) {
       title: item[1]
     })
   } else {
-    if (groupDepth > 3) {
-      console.error(
-        '[vuepress] detected a too deep nested sidebar group.'
-      )
-    }
     const children = item.children || []
     if (children.length === 0 && item.path) {
       return Object.assign(resolvePage(pages, item.path, base), {