[STLExtras] Remove incorrect hack to make indexed_accessor_range operator==...

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

This partially reverts c312f025

The motivation behind this is unclear and the change predates the clang
[implementation](https://github.com/llvm/llvm-project/commit/38b9d313e6945804fffc654f849cfa05ba2c713d)
of
[p2468r2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2468r2.html)
so I am not sure if it was ever intended to work. Rewritten template
operators were broken since the beginning.

Moreover, moving away from `friend` would be beneficial as these would
only accepted once clang revises its implementation for fixing
https://github.com/llvm/llvm-project/issues/70210. It also helps in
making sure that older compilers still compile LLVM (in C++20).