[libc++] Simplify the implementation of locale::id (#65781)

Since we use C++20 to build the dylib, we can use a lambda to do the
first-time initialization instead of emulating std::bind. This should
not change the behavior of the code at all, it merely simplifies it.

This removes a symbol from the dylib, however that symbol was only ever
used inside the dylib so it shouldn't break the ABI for anyone. I
confirmed that by searching for that symbol on the ABI boundary of a
large number of programs and couldn't find any references to that
function.