MAINTAINERS: remove kernel@stlinux.com obsolete mailing list

The STMicroelectronics dedicated mailing list kernel@stlinux.com
is no more available, remove it to avoid bouncing mails.

Several request to create a new mailing list has been send by Benjamin
Gaignard and me but without any answers.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Olof Johansson <olof@lixom.net>