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@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20241125064036.413536-1-lizhijian@xxxxxxxxxxx/
Signed-off-by: Moon Hee Lee <moonhee.lee.ca@xxxxxxxxx>
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