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/reaction.rs

@@ -900,7 +900,6 @@ Here's my footer -- bob@example.net"
             msg_header.as_bytes(),
             false,
             Some(100000),
-            false,
         )
         .await?
         .unwrap();
@@ -931,7 +930,6 @@ Here's my footer -- bob@example.net"
             msg_full.as_bytes(),
             false,
             None,
-            false,
         )
         .await?;