runtime(tutor): Make all tutor files utf-8 only

runtime(tutor): Make all tutor files utf-8 only

In this commit, all legacy encodings for tutor files are removed.

* All the tutor files `tutor1.xx` and `tutor2.xx` are now `utf-8`
* All `README.xx.txt` files are now `utf-8`
* All makefiles are removed, as they are no longer needed
* The `tutor.vim` script is simplified
* The `README` indicates `utf-8` as a requirement
* update vimtutor.bat file

In 2025, there is no realistic scenario where a beginner starts the Vim
9.1 tutor on a machine that lacks UTF-8 support.
This change will ease the maintenance of the tutor files. Cf #17562 for
an example.

closes: #17582
closes: #17586

Co-authored-by: RestorerZ <restorer@mail2k.ru>
Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: RestorerZ <restorer@mail2k.ru>
Signed-off-by: Christian Brabandt <cb@256bit.org>