$ sudo apt-get remove opendkim 

Uninstall opendkim including dependent package

If you would like to remove opendkim and it's dependent packages which are no longer needed from Ubuntu,

$ sudo apt-get remove --auto-remove opendkim 

Use Purging opendkim

If you use with purge options to opendkim package all the configuration and dependent packages will be removed.

$ sudo apt-get purge opendkim 

If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.

$ sudo apt-get purge --auto-remove opendkim 

See Also,