exec/cpu-all: Remove unused 'qemu/thread.h' header

Nothing is required from "qemu/thread.h" in "exec/cpu-all.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Message-Id: <20231212123401.37493-13-philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>