[TASK] Remove unused function BackendUtility::updatePagesTSconfig()

The function is not used in the core and marked as @internal since the
beginning of the time (revision 1). Therefore it is removed directly.

Resolves: #53471
Releases: 6.2
Change-Id: Ib74fa65b24caa9879b16921e13e3d0bed894b712
Reviewed-on: https://review.typo3.org/25228
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn