all: Convert folders to use filesystem abstraction

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4228

lib/osutil/hidden_unix.go

@@ -8,12 +8,4 @@
 
 package osutil
 
-func HideFile(path string) error {
-	return nil
-}
-
-func ShowFile(path string) error {
-	return nil
-}
-
 func HideConsole() {}