Remove pseudo-mechanism for workflow definition registration

Opencast workflow definitions are stored as XML files in etc/workflows.

There is an additional mechanism that suggests that the workflow configuration can be managed used the REST endpoint of the workflow service that offers those two functions:

• DELETE /workflow/definition/{id} to unregister a workflow definition
• PUT /workflow/definition to add a workflow definition

While those functions are implemented, they aren't actually useful as there is no persistence layer that would in any way ensure that the changes survive a restart of Opencast where Opencast would simply start over with the configuration found in etc/workflows.

Instead of using those functions, administrators can delete the workflow definition XML file in etc/workflows to unregister a workflow definition or change the file to update it.