Fix all outgoing messages popping up in selfchat (#2456)

Fix https://github.com/deltachat/deltachat-android/issues/1940, fix https://github.com/deltachat/deltachat-core-rust/issues/2220 (I assume these are the same bug)

The problem was:
- Gmail adds a header `Bcc: <self-address>` to our bcc-self message
- `to_id` was just set to the first recipient, in this case _self_
- it was seen that `to_id` is _self_, so it's a self-sent message

src/mimeparser.rs

@@ -1585,7 +1585,7 @@ fn get_attachment_filename(
 /// Returned addresses are normalized and lowercased.
 pub(crate) fn get_recipients(headers: &[MailHeader]) -> Vec<SingleInfo> {
     get_all_addresses_from_header(headers, |header_key| {
-        header_key == "to" || header_key == "cc" || header_key == "bcc"
+        header_key == "to" || header_key == "cc"
     })
 }