Draft: ci: Remove MESON_TESTTHREADS workaround
https://github.com/mesonbuild/meson/pull/13604 got merged and released with Meson 1.5.2, which we already use in the Docker images, so the workaround can be dropped.
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https://github.com/mesonbuild/meson/pull/13604 got merged and released with Meson 1.5.2, which we already use in the Docker images, so the workaround can be dropped.