api!: remove e2ee_enabled preference

The setting is already removed from the UIs,
but users who had it disabled previously have
no way to enable it. After this change
encryption is effectively always preferred.

src/mimeparser.rs

@@ -1519,7 +1519,7 @@ impl MimeMessage {
                 );
                 return Ok(false);
             }
-            Ok((key, _)) => key,
+            Ok(key) => key,
         };
         if let Err(err) = key.verify() {
             warn!(context, "Attached PGP key verification failed: {err:#}.");