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

[Benjamin Kramer via llvm-commits]

https://github.com/d0k approved this pull request.

Fine with me, but I suppose this will break older clangs just like e558be51bab051d1471d92e967f8a2aecc13567a did. Is everyone building clang in C++20 mode on a newer version now?

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