target/arm/ptw: Remove last uses of ptw->in_secure

Replace the last uses of ptw->in_secure with appropriate
checks on ptw->in_space.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230807141514.19075-10-peter.maydell@linaro.org