prepare_msg_raw: do not set GuaranteeE2ee

This code is inconsistent with EncryptHelper::should_encrypt, which uses
quorum rule.

UI does not display or use encryption state in-between preparing and
sending message anyway.

In addition, "sticky encryption" rule, which required all replies to
encrypted messages to be encrypted, is dropped. It is going to be
restored with the introduction of quoting.

python/tests/test_account.py

@@ -1067,6 +1067,7 @@ class TestOnlineAccount:
         # Majority prefers encryption now
         assert msg5.is_encrypted()
 
+    @pytest.mark.xfail(reason="Sticky encryption rule was removed")
     def test_reply_encrypted(self, acfactory, lp):
         ac1, ac2 = acfactory.get_two_online_accounts()