Remove exception notifications over email

Since these sites are no longer in active development and are fairly
stable, these notifications are more noise than anything useful.
Exceptions usually occur while folks attempt to exploit the site with
sending broken/erroneous payloads, which is not particularly
interesting.

Exceptions will continue to be logged to the production.log file and can
be examined on demand.

Should this functionality be needed, it must be introduced
conditionally, configured in the per-course-edition files.

Gemfile

@@ -38,10 +38,6 @@ gem 'sinatra', require: false
 # See https://stackoverflow.com/a/60491254/75715
 gem 'bigdecimal', '1.3.5'
 
-group :production do
-  gem 'exception_notification'
-end
-
 group :assets do
   gem 'sass-rails', '~> 4.0'
   gem 'sass', '~> 3.2.0'

Gemfile.lock

@@ -115,9 +115,6 @@ GEM
       mail (~> 2.2)
     erubis (2.7.0)
     eventmachine (1.0.8)
-    exception_notification (4.1.1)
-      actionmailer (>= 3.0.4)
-      activesupport (>= 3.0.4)
     execjs (2.7.0)
     factory_girl (4.5.0)
       activesupport (>= 3.0.0)
@@ -389,7 +386,6 @@ DEPENDENCIES
   diff-lcs
   draper
   email_spec
-  exception_notification
   factory_girl_rails
   faker
   guard

config/environments/production.rb

@@ -72,12 +72,6 @@
   config.action_mailer.smtp_settings       = config.smtp_settings.symbolize_keys
   config.action_mailer.default_url_options = {host: config.course_domain}
 
-  config.middleware.use ExceptionNotification::Rack, email: {
-    email_prefix: '[ERROR] ',
-    sender_address: "Exception Notifier <#{config.course_email}>",
-    exception_recipients: [config.course_email]
-  }
-
   # Eager load
   config.eager_load = true
 end