Chore: Remove dead code related to billing (#935)

- remove subscription model
- remove billing-related code

app/models/channel/web_widget.rb

@@ -2,16 +2,15 @@
 #
 # Table name: channel_web_widgets
 #
-#  id                 :integer          not null, primary key
-#  agent_away_message :string
-#  website_token      :string
-#  website_url        :string
-#  welcome_tagline    :string
-#  welcome_title      :string
-#  widget_color       :string           default("#1f93ff")
-#  created_at         :datetime         not null
-#  updated_at         :datetime         not null
-#  account_id         :integer
+#  id              :integer          not null, primary key
+#  website_token   :string
+#  website_url     :string
+#  welcome_tagline :string
+#  welcome_title   :string
+#  widget_color    :string           default("#1f93ff")
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#  account_id      :integer
 #
 # Indexes
 #