[libc++][spaceship][NFC] P1612R2: Mark remove `operator!=` from "Ranges...

[libc++][spaceship][NFC] P1612R2: Mark remove `operator!=` from "Ranges Library" items as "Complete"

Marked already implemented parts of P1612R2 as "Complete":
- `ranges::iota_view::iterator` https://reviews.llvm.org/D110774
- `iota_view::sentinel` https://reviews.llvm.org/D107396
- `filter_view::iterator` https://reviews.llvm.org/D109086
- `filter_view::sentinel` https://reviews.llvm.org/D109086
- `ranges::transform_view::iterator` https://reviews.llvm.org/D110774
- `transform_view::sentinel` https://reviews.llvm.org/D103056
- `take_view::sentinel` https://reviews.llvm.org/D123600
- `join_view::iterator` https://reviews.llvm.org/D107671
- `join_view::sentinel ` https://reviews.llvm.org/D107671
- `split_view::outer_iterator` https://reviews.llvm.org/D142063
- `split_view::inner_iterator` https://reviews.llvm.org/D142063

Note these operators were added and removed in C++20.

Reviewed By: Mordante, #libc

Differential Revision: https://reviews.llvm.org/D152721