Remove Peter Chubb as i/MX31 maintainer.

I'm leaving my current job and will no longer have access to the
hardware to test or maintain this port.

Signed-off-by: Peter Chubb <peter.chubb@data61.csiro.au>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>