users: reword ban notice messages.

* Show the ban length instead of the ban expiration date in ban notices.
* Fix the ban notice to not say "Your account has been temporarily
  banned" when it's a permanent ban.

app/presenters/user_presenter.rb

@@ -13,14 +13,6 @@ class UserPresenter
     user.created_at.strftime("%Y-%m-%d")
   end
 
-  def ban_reason
-    if user.is_banned?
-      "#{user.active_ban.reason}; expires #{user.active_ban.expires_at} (#{user.bans.count} bans total)"
-    else
-      nil
-    end
-  end
-
   def permissions
     permissions = []