target/riscv: Remove `NB_MMU_MODES` define

target/riscv: Remove `NB_MMU_MODES` define

Signed-off-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230306175230.7110-17-anjo@rev.ng>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>