MAINTAINERS: remove SPEAR entry

As the lastest spear directories are removed, delete the associated entry
in the MAINTAINERS file:
- arch/arm/cpu/arm926ejs/spear/
- arch/arm/include/asm/arch-spear/

Fixes: 570c3dcf ("arm: Remove spear600 boards and the rest of SPEAr support")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>