C#: Minor simplification.

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/File.qll

@@ -29,8 +29,7 @@ private module Impl = Make<Input>;
 
 class Container = Impl::Container;
 
-/** A folder. */
-class Folder extends Container, Impl::Folder { }
+class Folder = Impl::Folder;
 
 bindingset[flag]
 private predicate fileHasExtractionFlag(File f, int flag) {