[TASK] Rethink .txt files (GPL & Readme)

Removes few outdated Readme files.

Also brings the well-formatted official GPL.txt from
http://www.gnu.org/licenses/gpl-2.0.txt

Change-Id: Ie059d19fd2424f8626bcd7f0e5c040736fa31377
Resolves: #48397
Releases: 6.2
Reviewed-on: https://review.typo3.org/20931
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov