Do not add an error if the message is encrypted but not signed

Services like Lacre [1] on Disroot and Inbound Encryption on Posteo [2]
offer to encrypt all incoming messages with the provided OpenPGP
public key. Resulting messages are encrypted, but not end-to-end encrypted
and not signed by the sender, therefore should not have a padlock displayed.
However, such encrypted and unsigned message is also not an indication
of an error on ongoing attack, so we shoud not report this as a problem
to the user.

[1] https://lacre.io/
[2] https://posteo.de/en/help/how-do-i-activate-inbound-encryption-with-my-public-pgp-key

src/mimeparser.rs

@@ -391,11 +391,6 @@ impl MimeMessage {
         //         part.error = Some("Seems like DKIM failed, this either is an attack or (more likely) a bug in Authentication-Results checking. Please tell us about this at https://support.delta.chat.".to_string());
         //     }
         // }
-        if encrypted && parser.signatures.is_empty() {
-            for part in parser.parts.iter_mut() {
-                part.error = Some("No valid signature".to_string());
-            }
-        }
 
         if parser.is_mime_modified {
             parser.decoded_data = mail_raw;