Remove redirect_uri from internal OAuth app

Commit 6e7c21c allows us to have a NULL `redirect_uri`. Previously, this value was merely a placeholder due to the NOT NULL constraint.

Author: aaronskiba

lib/tasks/doorkeeper.rake

@@ -8,10 +8,6 @@ namespace :doorkeeper do
     ) do |a|
       a.scopes = 'read'
       a.confidential = true
-      # redirect_uri value is only used as a placeholder here (required by Doorkeeper).
-      # Tokens are minted server-side for already-authenticated first-party users.
-      # No redirect, authorization code, or third-party client is involved.
-      a.redirect_uri = "#{Rails.application.routes.url_helpers.root_url}oauth/callback"
     end
 
     puts "Internal OAuth app ready (id=#{app.id}, uid=#{app.uid})"