api!: remove save_mime_headers config option and dc_get_mime_headers()

This was only used in tests.
`msgs.mime_headers` coulmn remains
as it is used for HTML messages.

src/context.rs

@@ -919,12 +919,6 @@ impl Context {
             "show_emails",
             self.get_config_int(Config::ShowEmails).await?.to_string(),
         );
-        res.insert(
-            "save_mime_headers",
-            self.get_config_bool(Config::SaveMimeHeaders)
-                .await?
-                .to_string(),
-        );
         res.insert(
             "download_limit",
             self.get_config_int(Config::DownloadLimit)