fix: Just use shouldEncryptElement for the envelope "validation"

packages/moxxmpp/lib/src/xeps/xep_0384/xep_0384.dart

@@ -462,18 +462,8 @@ abstract class BaseOmemoManager extends XmppManagerBase {
       final envelopeChildren = envelope.firstTag('content')?.children;
       if (envelopeChildren != null) {
         children.addAll(
-          envelopeChildren
           // Do not add forbidden elements from the envelope 
-            .where((XMLNode child) {
-              for (final ignore in _doNotEncryptList) {
-                final xmlns = child.attributes['xmlns'] ?? '';
-                if (child.tag == ignore.tag && xmlns == ignore.xmlns) {
-                  return false;
-                }
-              }
-
-              return true;
-            }),
+          envelopeChildren.where(shouldEncryptElement),
         );
       } else {
         logger.warning('Invalid envelope element: No <content /> element');