selftests/mm: remove duplicate .gitignore entries

selftests/mm: remove duplicate .gitignore entries

Remove redundant entries in .gitignore confirmed by:

$ sort tools/testing/selftests/mm/.gitignore | uniq -d
hugetlb_dio
pkey_sighandler_tests_32
pkey_sighandler_tests_64

These entries were originally added by [1], and later duplicated by [2].

[1] https://lore.kernel.org/all/20240924185911.117937-1-lorenzo.stoakes@oracle.com/
[2] https://lore.kernel.org/all/20241125064036.413536-1-lizhijian@fujitsu.com/

Link: https://lkml.kernel.org/r/20250626020758.163243-1-moonhee.lee.ca@gmail.com
Signed-off-by: Moon Hee Lee <moonhee.lee.ca@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>