Cannot attach files to epics

Background

• The attach file button doesn't work when in the description.
• Also doesn't work when you try to drag a file into the description box while in edit mode.

Scope

• Simply remove the label Attach a file from the description text box so that it is clear that epics currently does not support adding files/images. That will be implemented separately in https://gitlab.com/gitlab-org/gitlab-ee/issues/3938.
• The drag-and-drop UI should not be functional. (Currently it is not. Something to verify after implementation of this issue.)