scripts: kernel-doc: Drop obsolete comments

What for? To improve the script maintainability.

1. License

As stated by Jonathan Corbet in the reply to my version 1, the SPDX line
is enough.

2. The to-do list comment

As suggested by Jonathan Corbet in reply to my version 3, this section
doesn't need to be transitioned. And so it is removed for clarity.

3. The historical changelog comments

As suggested by Jonathan Corbet in a reply to v3, this section can go.
I wanted to keep it, but since it doesn't contain copyright notices,
let's just have it clean and simple.

4. The "format of comments" comment block

As suggested by Jani Nikula in a reply to my first version of this
transformation, Documentation/doc-guide/kernel-doc.rst can serve as the
information hub for comment formatting. The section DESCRIPTION already
points there, so the original comment block can just be removed.

Suggested-by: Jonathan Corbet <corbet@lwn.net>
Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Tomasz Warniełło <tomasz.warniello@gmail.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Disliked-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/20220218181628.1411551-11-tomasz.warniello@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>