Removed file_io class as it is no longer necessary.

This was going to be used with the flatbuffer storage engine, but is obsolete
now that we're moving away from that and to LevelDB.

PiperOrigin-RevId: 307536025

Author: amablue

database/CMakeLists.txt

@@ -96,7 +96,6 @@ set(desktop_SRCS
     src/desktop/database_reference_desktop.cc
     src/desktop/disconnection_desktop.cc
     src/desktop/mutable_data_desktop.cc
-    src/desktop/persistence/file_io.cc
     src/desktop/persistence/in_memory_persistence_storage_engine.cc
     src/desktop/persistence/noop_persistence_manager.cc
     src/desktop/persistence/persistence_manager.cc