fix: revert the waitAny refactoring

[hargoniX]

This PR fixes a memleak caused by the Lean based IO.waitAny implementation by reverting it.

This the faulty Lean implementation:

def IO.waitAny (tasks : @& List (Task α)) (h : tasks.length > 0 := by exact Nat.zero_lt_succ _) :
    BaseIO α := do
  have : Nonempty α := ⟨tasks[0].get⟩
  let promise : IO.Promise α ← IO.Promise.new
  tasks.forM <| fun t => BaseIO.chainTask (sync := true) t promise.resolve
  return promise.result!.get

In a situation where we call this function repeatedly in a loop with a pair of tasks [t1, t2]
where t2 is a long lived task that we pass every time and t1 is fresh a short lived task, t2 will
accumlate more and more children from BaseIO.chainTask that fill memory over time. The old C++
implementation did not have this issue so we are reverting.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 19533ab1d4312903f98ff0429a5962db24ce402b --onto 3a42ee0c3060e2a8365927d205ead5990366053d. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-28 21:59:02)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 19533ab1d4312903f98ff0429a5962db24ce402b --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-28 21:59:04)