Index-free: Remove stale TODO (#740)

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteSchema.java

@@ -48,13 +48,6 @@ class SQLiteSchema {
   /**
    * The version of the schema. Increase this by one for each migration added to runMigrations
    * below.
-   *
-   * <p>TODO(index-free): The migration to schema version 9 doesn't backfill `read_time` as this
-   * requires rewriting the RemoteDocumentCache. For index-free queries to efficiently handle
-   * existing documents, we still need to populate read_time for all existing entries, drop the
-   * RemoteDocumentCache or ask users to invoke `clearPersistence()` manually. If we decide to
-   * backfill or drop the contents of the RemoteDocumentCache, we need to perform an additional
-   * schema migration.
    */
   static final int VERSION = 9;