http_server: stop fallback to WEBrick (#575)

fluent/fluentd#4899

Signed-off-by: Daijiro Fukuda <[email protected]>

Author: daipom

plugin-helper-overview/api-plugin-helper-http_server.md

@@ -2,7 +2,7 @@
 
 The `http_server` helper creates an HTTP server. This helper was introduced in v1.6.0.
 
-It supports [`async-http`](https://github.com/socketry/async-http)-based server to improve the performance. If `async-http` gem is not installed, this helper uses the standard [`webrick`](https://github.com/ruby/webrick) server instead.
+It supports [`async-http`](https://github.com/socketry/async-http)-based server to improve the performance.
 
 Here is an example: