installer: Remove "public domain" option in favor of CC-0

installer: Remove "public domain" option in favor of CC-0

Some people don't consider "public domain" to be a valid license
statement, so for new wikis it's better to avoid it entirely. CC-0 works
around all of those issues, so we should be encouraging that instead.

Also it's confusing to have two "public domain" options right next to
each other.

The asset needs to stay so existing wikis that picked "public domain"
don't break.

Bug: T122916
Change-Id: I3c380f79a4f27d4793b84602002f3db5c399ac32