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

@@ -1,10 +1,9 @@
-update cards
-set factor = 2500,
-    usn = ?,
-    mod = ?
-where factor != 0
-    and factor <= 2000
-    and (
-        did in DECK_IDS
-        or odid in DECK_IDS
+UPDATE cards
+SET factor = 2500,
+  usn = ?
+WHERE factor != 0
+  AND factor <= 2000
+  AND (
+    did IN DECK_IDS
+    OR odid IN DECK_IDS
   )

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(())
     }