download: Get rid of EPHY_DOWNLOAD_ACTION_DO_NOTHING

This differs from EPHY_DOWNLOAD_ACTION_NONE in that it explicitly
informs EphyDownload that it should not perform any download action.
Maybe that was actually important in the past, but nowadays there's
absolutely no difference, so just get rid of it.

https://bugzilla.gnome.org/show_bug.cgi?id=610586