Remove `jl_task_stack_buffer` (again) (#57116)
This was removed previously in PR #54527 but had to be reverted in PR
#54559 as one usage remained (more by accident then by design). This has
since then been resolved.
This is also part of PR #56477 but that seems stalled right now, and in
fact merging parts of it now may make the review of it easier later
on...
