MAINTAINERS: Remove Christophe Kerello from STM32MP entry

Christophe will not have maintainer activities, so remove its name.
Signed-off-by: NPatrice Chotard <patrice.chotard@st.com>
Cc: Christophe Kerello <christophe.kerello@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>

MAINTAINERS

@@ -295,7 +295,6 @@ F:	arch/arm/include/asm/arch-spear/
 
 ARM STM STM32MP
 M:	Patrick Delaunay <patrick.delaunay@st.com>
-M:	Christophe Kerello <christophe.kerello@st.com>
 M:	Patrice Chotard <patrice.chotard@st.com>
 L:	uboot-stm32@st-md-mailman.stormreply.com (moderated for non-subscribers)
 S:	Maintained