Fixusergroupfield

These are the housekeeping deletions of the merged PR https://github.com/akeeba/fof/pull/286. 

There were deletions in the form file to be confirmed and done before the PR was merged. The deletions were confirmed and the merge accepted. However, the deletions were not done, and so the commented out code and associated header file were left intact. 

This PR completes these intended deletions in form/field/usergroup.php; and, deletes form/header/usergroup.php.

Thank you for reviewing this PR!