MAINTAINERS: drop discontinued mailing list

The ST-internal Nomadik mailing list is going down. Remove
it from the MAINTAINERS file.

Cc: Olivier CLERGEAUD <olivier.clergeaud@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>