Remove Project Gutenberg header to make the file free

[jmkim]

Dear maintainers,

About the testdata _examples/Mark.Twain-Tom.Sawyer.txt,

The original text of "The Adventures of Tom Sawyer" by Mark Twain is in the public domain.

However, the eBook version from Project Gutenberg is non-free due to restrictions on commercial use, specifically clauses 1.E.7, 1.E.8, and 1.E.9 of the Project Gutenberg License.

To make this file free, this commit removes all references to the Project Gutenberg, in accordance with clause 1.C of the Project Gutenberg License.

Full text of the Project Gutenberg License:

• https://www.gutenberg.org/policy/license.html#the-full-project-gutenberg-license-in-legalese-normative

Legal references:

• https://lists.debian.org/debian-legal/2025/03/msg00004.html
• https://lists.debian.org/debian-devel/2025/03/msg00229.html
• https://lists.debian.org/debian-legal/2017/08/msg00001.html