Update absl::allocator_traits and absl::pointer_traits to always

use std::allocator_traits and std::pointer traits.

Note that the reason given in the comments for these implementations was
incorrect. Both of these exist in C++11, but not all compilers had
working implementations, so Abseil backfiled them. All supported compilers
now have working implementations.

https://en.cppreference.com/w/cpp/memory/allocator_traits
https://en.cppreference.com/w/cpp/memory/pointer_traits

Documentation has been updated to recommend the std:: spellings.

Fixes #1366

PiperOrigin-RevId: 503532746
Change-Id: Ia437f65a4c752581195dc582a41831b479d096c6