target/arm: Remove __attribute__((nonnull)) from ptw.c

target/arm: Remove __attribute__((nonnull)) from ptw.c

This was added in 7e98e21c098 as part of a reorg in which
one of the argument had been legally NULL, and this caught
actual instances.  Now that the reorg is complete, this
serves little purpose.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230620124418.805717-10-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>