selftests/mm: drop sys/dev test in uffd-stress test

With the new uffd unit test covering the /dev/userfaultfd path and syscall
path of uffd initializations, we can safely drop the devnode test in the
old stress test.

One thing is to avoid duplication of running the stress test twice which is
an overkill to only test the /dev/ interface in run_vmtests.sh.

The other benefit is now all uffd tests (that uses userfaultfd_open) can
run automatically as long as any type of interface is enabled (either
syscall or dev), so it's more likely to succeed rather than fail due to
unprivilege.

With this patch lands, we can drop all the "mem_type:XXX" handlings too.

Link: https://lkml.kernel.org/r/20230412164525.329176-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Safonov <0x7f454c46@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>