bsp: Remove N1SDP support

meta-arm has removed support for EOL N1SDP on master:
https://git.yoctoproject.org/meta-arm/commit/ci?id=6db139e1fd7b724c7a5caacb07a4151411be877f



Therefore remove the platform from meta-cassini-bsp,
update the CI files and documentation.

Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>