Removed immutable type setting for latest security price

For whatever reasons, there are actually some referenced latest security
prices in files out there in the wild.

Issue: #4117
Issue: https://forum.portfolio-performance.info/t/fehlermeldungen-nach-update-auf-version-70/29289
Issue: https://forum.portfolio-performance.info/t/fehler-beim-offnen-xml-kann-nicht-geparst-werden/21859/18
Issue: https://www.wertpapier-forum.de/topic/38306-portfolio-performance-mein-neues-programm/?do=findComment&comment=1728577

name.abuchen.portfolio/src/name/abuchen/portfolio/model/ClientFactory.java

@@ -1730,7 +1730,6 @@ private static synchronized XStream xstream()
             xstream.alias("security", Security.class);
             xstream.addImplicitCollection(Security.class, "properties");
             xstream.alias("latest", LatestSecurityPrice.class);
-            xstream.addImmutableType(LatestSecurityPrice.class, false);
             xstream.alias("category", Category.class); // NOSONAR
             xstream.alias("watchlist", Watchlist.class);
             xstream.alias("investment-plan", InvestmentPlan.class);