Fix errors for Clang 16

[thejohnfreeman]

• std::{u,bi}nary_function were removed in C++17. They were empty classes with a few associated types. We already have conditional code to define the types. Just make it unconditional.
• libc++ checks a cast in an unevaluated context to see if a type inherits from a binary function class in the standard library, e.g. std::equal_to, and this causes an error when the type privately inherits from such a class. Change these instances to public inheritance.
• We don't need a middle-man for the empty base optimization. Prefer to inherit directly from an empty class than from beast::detail::empty_base_optimization.
• Clang warns when all the uses of a variable are removed by conditional compilation of assertions. Add a [[maybe_unused]] annotation to suppress it.
• As a drive-by clean-up, remove commented code.

See related work in #4486.

[HowardHinnant]

I recommend changing this commit message to:
[FOLD] Format code

[thejohnfreeman]

I expect this whole PR to be squashed into one commit, with the message subject line matching the PR title and message body matching the first comment.

[seelabs]

👍