Remove unused code

In the commit: 315c57929a this code was added, but at that time this field
was no longer in use, even though it existed in the database.

I don't know why this line was added, but there seems to be no point in
keeping it.

app/controllers/management/users_controller.rb

@@ -59,7 +59,6 @@ class Management::UsersController < Management::BaseController
       @user.confirmed_at = Time.current
 
       @user.newsletter = false
-      @user.email_on_proposal_notification = false
       @user.email_digest = false
       @user.email_on_direct_message = false
       @user.email_on_comment = false