[TASK] Revise typo3/sysext/README.txt

* Replace GIT repository link with the new one
* Remove information about submodules

Change-Id: I60db762ce831c7edea413d9f7780a5617b7f9e96
Resolves: #50410
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/22556
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters