config.h: Remove unused HAVE_LIBSDL2_MIXER define #4129

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4 warnings and 2 notices

`$HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK` is set: not checking for outdated dependents or dependents with broken linkage!

pkgconf 2.5.1 is already installed and up-to-date. To reinstall 2.5.1, run: brew reinstall pkgconf

ninja 1.13.1 is already installed and up-to-date. To reinstall 1.13.1, run: brew reinstall ninja

python@3.13 3.13.5 is already installed and up-to-date. To reinstall 3.13.5, run: brew reinstall python@3.13

The macos-latest label will migrate to macOS 15 beginning August 4, 2025. For more information see https://github.com/actions/runner-images/issues/12520