Remove mail-notify

This Gem is no longer being used so we can safely remove it.

Gemfile

@@ -43,7 +43,6 @@ gem "govuk_design_system_formbuilder"
 gem "govuk_markdown"
 gem "jsonb_accessor"
 gem "jwt"
-gem "mail-notify"
 gem "notifications-ruby-client"
 gem "okcomputer"
 gem "omniauth_openid_connect"

Gemfile.lock

@@ -336,13 +336,6 @@ GEM
       net-imap
       net-pop
       net-smtp
-    mail-notify (2.0.0)
-      actionmailer (>= 5.2.8.1)
-      actionpack (>= 5.2.8.1)
-      actionview (>= 5.2.8.1)
-      activesupport (>= 5.2.8.1)
-      notifications-ruby-client (~> 6.0)
-      rack (>= 2.1.4.1)
     marcel (1.0.4)
     matrix (0.4.2)
     mechanize (2.12.2)
@@ -739,7 +732,6 @@ DEPENDENCIES
   jsbundling-rails
   jsonb_accessor
   jwt
-  mail-notify
   mechanize
   notifications-ruby-client
   okcomputer