[TASK] Remove TCA building aspect and move code to bootstrap

The patch moves the slot code that implements the override loading
logic back to the bootstrap but keeps the signal.

Resolves: #59770
Related: #59414
Related: #59468
Releases: 6.3, 6.2
Change-Id: Ib4691eaa94f258ebd96f2609fb267cadc8356090
Reviewed-on: https://review.typo3.org/31234
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring