Deletes the specified custom user settings for a single user.
###Deleting Grouped Custom User Settings###
If the custom user settings you want to delete are grouped, you must include the following information in the header, after Content-Type, in the request:
group_name is your designated name for the group of customer user settings.
If the extra header information is not included, only the custom user settings that were added without a group are deleted.
The external account number (int) or account ID Guid.