MAINTAINERS: tegra: Remove self as maintainer

I have not been doing any Tegra maintainance work for a while, and this
is unlikely to change in the foreseeable future. Better reflect the
actual state of things by removing myself from the list.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>