feat: Don't download group messages unconditionally

There was a comment that group messages should always be downloaded to avoid inconsistent group
state, but this is solved by the group consistency algo nowadays in the sense that inconsistent
group state won't spread to other members if we send to the group. Moreover, encrypted messages are
now always downloaded, and unencrypted chat replies too, and as for ad-hoc groups,
`Config::ShowEmails` controls everything.

src/imap.rs

@@ -27,7 +27,7 @@ use crate::calls::{create_fallback_ice_servers, create_ice_servers_from_metadata
 use crate::chat::{self, ChatId, ChatIdBlocked, add_device_msg};
 use crate::chatlist_events;
 use crate::config::Config;
-use crate::constants::{self, Blocked, Chattype, ShowEmails};
+use crate::constants::{self, Blocked, ShowEmails};
 use crate::contact::{Contact, ContactId, Modifier, Origin};
 use crate::context::Context;
 use crate::events::EventType;
@@ -2229,21 +2229,6 @@ pub(crate) fn create_message_id() -> String {
     format!("{}{}", GENERATED_PREFIX, create_id())
 }
 
-/// Returns chat by prefetched headers.
-async fn prefetch_get_chat(
-    context: &Context,
-    headers: &[mailparse::MailHeader<'_>],
-) -> Result<Option<chat::Chat>> {
-    let parent = get_prefetch_parent_message(context, headers).await?;
-    if let Some(parent) = &parent {
-        return Ok(Some(
-            chat::Chat::load_from_db(context, parent.get_chat_id()).await?,
-        ));
-    }
-
-    Ok(None)
-}
-
 /// Determines whether the message should be downloaded based on prefetched headers.
 pub(crate) async fn prefetch_should_download(
     context: &Context,
@@ -2262,14 +2247,6 @@ pub(crate) async fn prefetch_should_download(
     // We do not know the Message-ID or the Message-ID is missing (in this case, we create one in
     // the further process).
 
-    if let Some(chat) = prefetch_get_chat(context, headers).await? {
-        if chat.typ == Chattype::Group && !chat.id.is_special() {
-            // This might be a group command, like removing a group member.
-            // We really need to fetch this to avoid inconsistent group state.
-            return Ok(true);
-        }
-    }
-
     let maybe_ndn = if let Some(from) = headers.get_header_value(HeaderDef::From_) {
         let from = from.to_ascii_lowercase();
         from.contains("mailer-daemon") || from.contains("mail-daemon")