mm: cleanup: remove unused tsk arg from __access_remote_vm

Despite a comment that said that page fault accounting would be charged to
whatever task_struct* was passed into __access_remote_vm(), the tsk
argument was actually unused.

Making page fault accounting actually use this task struct is quite a
project, so there is no point in keeping the tsk argument.

Delete both the comment, and the argument.

[rppt@linux.ibm.com: changelog addition]

Link: https://lkml.kernel.org/r/20201026074137.4147787-1-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>