avoid bumping mtime when correcting card eases on upgrade

The USN is still set, which should cause the cards to sync in the
non-conflict case, but if changes have been made on other devices
the ease fix will not take priority, as we could be overwriting the
reviews of someone who has not synced in a while.

rslib/src/storage/card/fix_low_ease.sql

@@ -1,7 +1,6 @@
 UPDATE cards
 SET factor = 2500,
-  usn = ?,
-  mod = ?
+  usn = ?
 WHERE factor != 0
   AND factor <= 2000
   AND (

rslib/src/storage/card/mod.rs

@@ -404,9 +404,7 @@ impl super::SqliteStorage {
         ids_to_string(&mut ids, &affected_decks);
         let sql = include_str!("fix_low_ease.sql").replace("DECK_IDS", &ids);
 
-        self.db
-            .prepare(&sql)?
-            .execute(params![self.usn(server)?, TimestampSecs::now()])?;
+        self.db.prepare(&sql)?.execute(params![self.usn(server)?])?;
 
         Ok(())
     }