Declassify on move only

app/src/main/java/eu/faircode/email/Core.java

@@ -1183,6 +1183,7 @@ class Core {
             throw new IllegalArgumentException("not selectable type=" + target.type);
 
         // De-classify
+        if (!copy)
             for (EntityMessage message : messages)
                 MessageClassifier.classify(message, folder, target, context);