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

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: #62165
Related: #61957
Releases: master
Change-Id: Id4723c5c366d629fabb114456eebbed746aa9a53
Reviewed-on: http://review.typo3.org/33267
Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>