P2869R4: Remove Deprecated `shared_ptr` Atomic Access APIs from C++26

[ldionne]

Link: https://wg21.link/P2869R4

[Zingam]

This was submitted:

• [libc++] Deprecated shared_ptr Atomic Access APIs as per P0718R2 & Implemented P2869R3: Remove Deprecated shared_ptr Atomic Access APIs from C++26 #87111
  and later reverted by:
• Revert "[libc++] Deprecated shared_ptr Atomic Access APIs as per P0… #92920
  I'll resubmit it as requested again later.

[frederick-vs-ja]

Looks like that we should proceed on this after implementing P0718R2 (#99980).

[Zingam]

Looks like that we should proceed on this after implementing P0718R2 (#99980).

This was already implemented. The idea is to resubmit it as two PRs to be merged whenever appropriate.