[libc++][spaceship][NFC] P1612R2: Mark some items "remove `operator!=` from `*_iterat...

[libc++][spaceship][NFC] P1612R2: Mark some items "remove `operator!=` from `*_iterator`" as "Complete"

Several parts of P1612R2 are already implemented:
- `common_iterator` (new in C++20) https://reviews.llvm.org/D103335
- `unreachable_sentinel_t` (new in C++20) https://reviews.llvm.org/D107920
- `istream_iterator` (removed) https://reviews.llvm.org/D119620
- `istreambuf_iterator` (removed) https://reviews.llvm.org/D119620

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D152656