C++17 is the project-wide default since f9057cea51 ("fix(FTBFS):
meson: raise C++ standard to C++17"), so let's drop these local
overrides.
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23048>