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

RELEASE-NOTES-1.28

@@ -106,6 +106,8 @@ changes to languages because of Phabricator reports.
 * DifferenceEngine::generateDiffBody() was removed (deprecated since 1.21).
 * UploadBase::stashFileGetKey() and UploadBase::stashSession() were deprecated.
   Use ...->stashFile()->getFileKey() instead.
+* "Public domain" was removed as a wiki license option from the installer, in
+  favour of CC-0.
 
 == Compatibility ==
 

includes/installer/Installer.php

@@ -294,10 +294,6 @@ abstract class Installer {
 			'url' => 'https://creativecommons.org/publicdomain/zero/1.0/',
 			'icon' => '$wgResourceBasePath/resources/assets/licenses/cc-0.png',
 		],
-		'pd' => [
-			'url' => '',
-			'icon' => '$wgResourceBasePath/resources/assets/licenses/public-domain.png',
-		],
 		'gfdl' => [
 			'url' => 'https://www.gnu.org/copyleft/fdl.html',
 			'icon' => '$wgResourceBasePath/resources/assets/licenses/gnu-fdl.png',

resources/assets/licenses/README

@@ -0,0 +1,4 @@
+These license icons are used in LocalSettings.php files that are generated by
+the installer. Although public domain has been removed from the installer as
+an option, the image needs to remain here to support installations which refer
+to it in LocalSettings.php.