in_monitor_agent: remove needless require (#4898)

**Which issue(s) this PR fixes**: 
* Partially Fixes #4648

**What this PR does / why we need it**: 
Stop requiring WEBrick on `in_monitor_agent`. 

Even if async is not installed, http_server helper requires WEBrick on
its own.
So, there is no need to require WEBrick here.

There is no impact on the specification. (This is just code
refactoring.)

**Docs Changes**:
Not needed.

**Release Note**: 
Not needed. (This is just code refactoring.)

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

lib/fluent/plugin/in_monitor_agent.rb

@@ -15,7 +15,6 @@
 #
 
 require 'json'
-require 'webrick'
 require 'cgi'
 
 require 'fluent/config/types'