removed useless domain parameter to Slack module

The Slack module was updated due to a change in the Slack API. 

https://github.com/ansible/ansible-modules-extras/commit/513eadbd9289c013127b562a61274f56a768edd4#diff-bd8d05bbd5b603181d453fa3bb1ec879

This change left the `domain` parameter as a required parameter - but the parameter was never used.

For 1.9, I propose to remove the parameter and update the documentation accordingly.