Documentation: Remove unused "nps_mtm_hs_ctr" from kernel-parameters.txt

The "nps_mtm_hs_ctr" parameter has been removed in commit dd7c7ab0
("ARC: [plat-eznps]: Drop support for EZChip NPS platform"). Remove it
from the documentation now, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240614190804.602970-1-thuth@redhat.com