Remove "public domain" from license options in Save Online ...
Simply scrapping public-domain as possible option is not the right solution though as there will be existing work on musescore.com under this license. So in case a score is edited (and saved online) which has this license option, the option should be listed and selected in the license options list. In case the work has not been saved online yet, or has another license selected, the public domain mark should not be listed.
While on this issue, also update the wording to be on par with the licensing terms on musescore.com.
Once applied to master, this issue can to be backported to 2.1.