Milestones should not be set on epics (issue promotion)

When we promote an issue to an epic, we copy a milestone from the issue to the new epic (https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/services/issuable/clone/attributes_rewriter.rb#L13).

We, however don't support milestones on epics (yet?). Therefore we shouldn't copy the milestone for epics.

If we decide we don't want to have milestones on epics at all we should probably consider removing the field.