Lately there's been issues where "public domain" isn't considered to be a proper license statement (e.g. http://wiki.spdx.org/view/Legal_Team/Decisions/Dealing_with_Public_Domain_within_SPDX_Files, etc.).
Since we already provide CC-0 as a license option, we should remove public domain. Plus it's a little confusing to have two public domain options right next to each other.