[TASK] Remove todo in AbstractDomainObject

A todo for the `$uid` property in AbstractDomainObject states,
that a type declaration should be introduced in v13. This has been
achieved with #102878, so the todo can be removed.

Resolves: #103460
Related: #102878
Releases: main
Change-Id: I96535dc78e1568198c70d39143e7c27e0ed1ce4c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83563


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>