include/unordered_{map,set}: remove, use `std` directly instead

include/unordered_{map,set}: remove, use `std` directly instead

This is only an alias for the `std` classes anyway.  This effectively
reverts commit 8e8672047ccc which added these aliases, probably
because C++11 was new and not all compilers supported it properly, but
that was 12 years ago and we're onl C++20 meanwhile.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>