buffer.c: evaporate overlays in all indirect buffers
This fixes bug#58928. The patch works by moving the
`evaporate_overlays` check closer to `itree_delete_gap`.
* src/buffer.c (adjust_overlays_for_delete_in_buffer): New function,
integrating the functionality of `evaporate_overlays`.
(adjust_overlays_for_delete): Use it.
(evaporate_overlays): Delete function.