[TASK] Cleanup ChangeLog

Remove mistakenly introduced submodule history in 6.2.0alpha1
release.

Resolves: #49110
Releases: 6.2
Change-Id: I00cb44c5ff718a2bf53f61c5ce7cdf778c783325
Reviewed-on: https://review.typo3.org/21403
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny