MAINTAINERS: Remove Christophe Kerello from STM32MP entry

Christophe will not have maintainer activities, so remove its name.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Cc: Christophe Kerello <christophe.kerello@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>