remove unused code

lib/models/ente_file.dart

@@ -5,8 +5,4 @@ abstract class EnteFile {
   // returns cacheKey which should be used while caching entry related to
   // this file.
   String? cacheKey();
-
-  // returns localIdentifier for the file on the host OS.
-  // Can be null if the file only exist on remote
-  String? localIdentifier();
 }

lib/models/file.dart

@@ -275,13 +275,9 @@ class File extends EnteFile {
   }
 
   @override
+  //add nullable return type when migrating to null safety
   String cacheKey() {
     // todo: Neeraj: 19thJuly'22: evaluate and add fileHash as the key?
     return localID ?? uploadedFileID?.toString() ?? generatedID?.toString();
   }
-
-  @override
-  String localIdentifier() {
-    return localID;
-  }
 }