Modernize equality operators in //base (3/3)

This CL is pure clean-up and contains no functionality changes. It does
the following:
- Remove unneeded operator!= declarations/definitions since C++20 can
  automatically derive those from operator==.
- Default operator== where this is equivalent to the current behavior.
- Default operator<=> where this is equivalent to the current
  behavior.

Bug: 40256175

Change-Id: I369d99be1dec2c71c99f2a4c5c1990c4b2473940
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532711
Auto-Submit: Jan Keitel <jkeitel@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459285}