DEV: Drop webrick gem since it is already available in Discourse core (#123)

This makes upgrading the webrick gem in Discourse core much easier since
we will no longer have conflicting gem versions.

Author: tgxworld

lib/collector_demon.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 #
 require_dependency "demon/base"
+require "webrick"
 
 class DiscoursePrometheus::CollectorDemon < ::Demon::Base
   def self.prefix

plugin.rb

@@ -10,7 +10,6 @@
 module ::DiscoursePrometheus
 end
 
-gem "webrick", "1.9.1"
 gem "prometheus_exporter", "2.0.6"
 
 require "prometheus_exporter/client"