Do not validate detached signature for unencrypted messages

[link2xt]

Here we validate detached signature for mail (not decrypted_mail):

core/src/mimeparser.rs

Lines 484 to 489 in a89b632

	let mail = mail.as_ref().map(|mail| {
	let (content, signatures_detached) = validate_detached_signature(mail, &public_keyring)
	.unwrap_or((mail, Default::default()));
	signatures.extend(signatures_detached);
	content
	});

If the signature is valid, we put it into signatures set, but according to its documentation non-empty signatures set means that the message is encrypted and signed:

core/src/mimeparser.rs

Lines 79 to 84 in a89b632

	/// Set of valid signature fingerprints if a message is an
	/// Autocrypt encrypted and signed message.
	///
	/// If a message is not encrypted or the signature is not valid,
	/// this set is empty.
	pub signatures: HashSet<Fingerprint>,

If the messages is just signed with a detached signature, but not encrypted, it should be treated as insecure message like we do for normal signatures.

Noticed this while re-reading the code that validates signatures, opened a related refactoring PR: #7119

[iequidoo]

This never was a bug because later in this function there's the code:

        if !is_encrypted {
            signatures.clear();
        }

EDIT: And this happens before the construction of MimeMessage for which the signatures field is documented.