gluon-legacy: convert the remaining legacy infrastructure to a simple invariant script

package/gluon-legacy/files/lib/gluon/legacy/legacy-upgrade

-#!/usr/bin/lua
-
-local site = require 'gluon.site_config'
-
-local ret = 1
-
-for _, file in ipairs(site.legacy.version_files) do
-	if os.remove(file) then
-		ret = 0
-	end
-end
-
-os.exit(ret)

package/gluon-legacy/files/lib/gluon/legacy/legacy-upgrade-late

-#!/usr/bin/lua
-
-local site = require 'gluon.site_config'
-
-
-for _, file in ipairs(site.legacy.old_files) do
-	os.remove(file)
-end

package/gluon-legacy/files/lib/gluon/upgrade/core/invariant/990-legacy-late

+#!/usr/bin/lua
+
+local site = require 'gluon.site_config'
+local sysconfig = require 'gluon.sysconfig'
+
+
+if sysconfig.gluon_version == 'legacy' then
+  for _, file in ipairs(site.legacy.old_files) do
+    os.remove(file)
+  end
+end