Modernize equality operators

This CL is pure clean-up and contains no functionality changes.
Depending on the files covered in the CL (since the CL was generated
using git cl split), it does a subset of 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.

This CL was uploaded by git cl split.

R=xiyuan@chromium.org

Bug: 40256175
Change-Id: I08c92e6e2d83225cf6dc87e549cab9ce0c113553
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6512944
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456411}

components/account_id/account_id.cc

@@ -83,10 +83,6 @@ bool AccountId::operator==(const AccountId& other) const {
   }
 }
 
-bool AccountId::operator!=(const AccountId& other) const {
-  return !operator==(other);
-}
-
 bool AccountId::operator<(const AccountId& right) const {
   // TODO(alemate): update this once all AccountId members are filled.
   return user_email_ < right.user_email_;

components/account_id/account_id.h

@@ -47,7 +47,6 @@ class COMPONENT_EXPORT(COMPONENTS_ACCOUNT_ID) AccountId {
   // If both are not UNKNOWN and not equal then it returns false.
   // If AccountType == GOOGLE then it checks if either ids or emails are equal.
   bool operator==(const AccountId& other) const;
-  bool operator!=(const AccountId& other) const;
   bool operator<(const AccountId& right) const;
 
   bool empty() const;