chore: remove plugin-clean-urls (#1464)

@vuepress/plugin-clean-urls was moved to https://github.com/vuepress/vuepress-plugin-clean-urls

Author: shigma

packages/@vuepress/plugin-clean-urls/.npmignore

@@ -37,7 +37,7 @@ And providing you are in `foo/one.md`:
 
 ### Redirection for URLs <Badge text="1.0.0-alpha.37"/>
 
-VuePress supports redirecting to clean links. If a link `/foo` is not found, VuePress will look for a existing `/foo/` or `/foo.html`. Conversely, when one of `/foo/` or `/foo.html` is not found, VuePress will also try the other. With this feature, we can customize your website's urls with the official plugin [@vuepress/plugin-clean-urls](../plugin/official/plugin-clean-urls.md).
+VuePress supports redirecting to clean links. If a link `/foo` is not found, VuePress will look for a existing `/foo/` or `/foo.html`. Conversely, when one of `/foo/` or `/foo.html` is not found, VuePress will also try the other. With this feature, we can customize your website's urls with the official plugin [vuepress-plugin-clean-urls](https://vuepress.github.io/plugins/clean-urls/).
 
 ::: tip
 Regardless of whether the permalink and clean-urls plugins are used, your relative path should be defined by the current file structure. In the above example, even though you set the path of `/foo/one.md` to `/foo/one/`, you should still access `/foo/two.md` via `./two.md`.

packages/@vuepress/plugin-clean-urls/README.md

@@ -37,7 +37,7 @@
 
 ### 链接的重定向 <Badge text="1.0.0-alpha.37"/>
 
-VuePress 支持重定向到干净链接。如果一个链接 `/foo` 找不到，VuePress 会自行寻找一个可用的 `/foo/` 或 `/foo.html`。反过来，当 `/foo/` 或 `/foo.html` 中的一个找不到时，VuePress 也会尝试寻找另一个。借助这种特性，我们可以通过官方插件 [@vuepress/plugin-clean-urls](../plugin/official/plugin-clean-urls.md) 定制你的网站路径。
+VuePress 支持重定向到干净链接。如果一个链接 `/foo` 找不到，VuePress 会自行寻找一个可用的 `/foo/` 或 `/foo.html`。反过来，当 `/foo/` 或 `/foo.html` 中的一个找不到时，VuePress 也会尝试寻找另一个。借助这种特性，我们可以通过官方插件 [vuepress-plugin-clean-urls](https://vuepress.github.io/plugins/clean-urls/) 定制你的网站路径。
 
 ::: tip 注意
 无论是否使用了 permalink 和 clean-urls 插件，你的相对路径都应该依赖于当前的文件结构来定义。在上面的例子中，即使你将 `/foo/one.md` 的路径设为了 `/foo/one/`，你依然应该通过 `./two.md` 来访问 `/foo/two.md`。