Remove `sync_seen_flags()` eventually

[link2xt]

This is a follow-up for #7005
It depends on CONDSTORE which is still not supported in maddy: foxcpp/maddy#727
This mechanism is replaced with sending MDNs to self in #7738
We will need to wait for this change to be released and users to update first, the issue is just here so we don't forget.
CONDSTORE should also be removed from https://github.com/chatmail/core/blob/main/standards.md.
store_seen_flags_on_imap can be kept for unencrypted non-chat messages if it's not much code.

[adbenitez]

btw, will the MDN be also sent to self even if user has disabled read receipts? I hope so because otherwise this will break sync

[link2xt]

Yes, MDN is sent to self even if read receipts are disabled, but multi-device mode is on:

core/src/message.rs

Lines 1939 to 1941 in 2531dfe

	} else if context.get_config_bool(Config::BccSelf).await? {
	Some(ContactId::SELF)
	} else {

[iequidoo]

We must also get rid of the \Seen flag handling in receive_imf for encrypted messages, it mustn't be trusted. For unencrypted messages it makes sense, but need to review the corresponding code.