RTC attachment deletion
One answer
If your requirement is deleting the attachment from RTC repository, according to the Enhancement 135695: Allow to delete attachments in the Web UI you can not delete the attachment from web client. However you can delete the attachment from RTC Eclipse client by opening the workitem, right-click the attachment from Links tab and select "Delete".
Comments
RTC 6.0.x provides a trashbin (remove) and an X being the attachment (shows delete). this is odd. To make sure the attachment is really deleted, I would use the Eclipse client. Also see https://jazz.net/library/article/1459.