selftests/mm: remove flaky header check

Commit 96ed62ea0298 ("mm: page_frag: fix a compile error when kernel is
not compiled") introduced a check to avoid attempting to build the
page_frag module if <linux/page_frag_cache.h> is missing.

Unfortunately this check only works if KDIR points to /lib/modules/... or
an in-tree kernel build. It always fails if KDIR points to an out-of-tree
build (i.e. when the kernel was built with O=... make) because only
generated headers are present under $KDIR/include/ in that case.

A recent commit switched KDIR to default to the kernel's build directory,
so that check is no longer justified.

Link: https://lkml.kernel.org/r/20260122170224.4056513-3-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Usama Anjum <Usama.Anjum@arm.com>
Cc: wang lian <lianux.mm@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>