[!!!][TASK] Remove old BE implementation of indexed_search

The old code can be removed as the new module has been merged.

The breaking change is already documented in the file
'Breaking-61510-IndexedSearch.rst'

Change-Id: Ief1867cbaa2f0768e3e8c08c490e64963b91a06c
Resolves: #61638
Releases: master
Reviewed-on: http://review.typo3.org/35338
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>