fix: disable fetch_existing_msgs setting by default

This caused too many problems after switching
the default setting for `show_emails`
from DC_SHOW_EMAILS_OFF to DC_SHOW_EMAILS_ALL
in <#4019>

There is a topic <https://support.delta.chat/t/setting-no-chats-only-for-show-classic-e-mails-showing-classic-emails/2481>
on the forum with multiple requests to revert this setting
due to old emails being downloaded.

Author: link2xt

src/config.rs

@@ -145,7 +145,7 @@ pub enum Config {
     /// If set to "1", on the first time `start_io()` is called after configuring,
     /// the newest existing messages are fetched.
     /// Existing recipients are added to the contact database regardless of this setting.
-    #[strum(props(default = "1"))]
+    #[strum(props(default = "0"))]
     FetchExistingMsgs,
 
     /// If set to "1", then existing messages are considered to be already fetched.