nilfs2: MAINTAINERS: drop unreachable project mirror site

The hosting site where the nilfs project had a mirror site continues to be
in trouble, so we have decided not to use that site.  This will reflect it
in the MAINTAINERS file.

Link: https://lkml.kernel.org/r/20240208093018.6334-1-konishi.ryusuke@gmail.com


Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>