The domain registration comes as a package with domain mapping. It's not possible to remove the domain mapping once you have a domain registered.
To point the domain you registered at WordPress.com to another server (hosting service), please follow the instructions here:
http://en.support.wordpress.com/domain-mapping/domain-management/#update-nameservers
Note you will need to obtain the name server values from your current host. You should be able to find them on their support documentation, or contact them and ask for the information. (Name servers may be referred as "nameservers" or "NS records" instead.)
Hope this helps - let us know if you have any more questions!