test: remove fetch_existing tests

fetch_existing option is not enabled in existing clients
and does not work with encrypted messages
without importing the key into a newely created account.

src/config.rs

@@ -182,12 +182,6 @@ pub enum Config {
     #[strum(props(default = "0"))] // also change MediaQuality.default() on changes
     MediaQuality,
 
-    /// 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 = "0"))]
-    FetchExistingMsgs,
-
     /// If set to "1", then existing messages are considered to be already fetched.
     /// This flag is reset after successful configuration.
     #[strum(props(default = "1"))]
@@ -707,7 +701,6 @@ impl Context {
             | Config::SentboxWatch
             | Config::MvboxMove
             | Config::OnlyFetchMvbox
-            | Config::FetchExistingMsgs
             | Config::DeleteToTrash
             | Config::Configured
             | Config::Bot