compress/bzip2/testdata: make Mark.Twain-Tom.Sawyer.txt free

Commit 7a1fb95d strips non-free license
from Mark.Twain-Tom.Sawyer.txt, but forgot to remove it from the compressed
version of the file.

Update #13216

Change-Id: I60f53275d56ba5baa6898db47b1d41f85e985c00
Reviewed-on: https://go-review.googlesource.com/17264Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>