BUG: remove new action option --upgrade-epd.

The function of `--upgrade-epd` is the same as `enpkg epd`, but as implemented it did not behave as other installs do (prompting for login) when user is not logged in, which can lead to a confusing exception traceback.

This could be fixed, but why do so when this action option is not necessary?

The important related feature of PR #72, which included `--upgrade-epd`, was the removal of epd from the list of normal packages displayed in `--whats-new` and installed with `--update-all`. But doing this does not require a separate `--upgrade-epd` action option. One might consider replacing `enpkg epd` with `--upgrade-epd`, but the latter did not support installing old versions of epd, only the latest. Installing an old version can be useful when troubleshooting.