[llvm] [STLExtras] Remove incorrect hack to make indexed_accessor_range operator== compatible with C++20 (PR #72220)

[Ilya Biryukov via llvm-commits]

ilya-biryukov wrote:

Both internal and buildbot builds should be fine after this change.
And they should keep working after #72213 lands because it does some extra checks to report those cases as warnings rather than errors (hence, won't break the builds either).

LGTM!

https://github.com/llvm/llvm-project/pull/72220