[TASK] ext:cms Remove some obsolete files

* readme.txt: Old information, nobody overloads cms anyway
* tslib/GPL.txt: GPL.txt from root directory catches this
* tslib/README.txt: Only obsolete information
* tslib/index.html: Leftover when tslib was linked from root

Change-Id: Ibfb2ac039b7d48621d2a6e2050b42a127c3c4662
Resolves: #45185
Releases: 6.1
Reviewed-on: https://review.typo3.org/18033
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn