[TASK] Remove the "@todo define visibility" in ext:core

The "@todo Define visibility" is not very helpful because:

 * Nobody knows what breaks if changing the visibility
 * The code is flooded with todos which makes it hard
   to work with in an IDE

During ACME it was decided to remove the @todo again.

Resolves: #62178
Related: #61957
Releases: master
Change-Id: I814739b00128a8ce7403abb42338b10dd598c8c4
Reviewed-on: http://review.typo3.org/33275
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>