mm: Drop reliable_reserve_size

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4SK3S


CVE: NA

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Commit 368d710d ("mm: Fallback to non-mirrored region below low watermark")
already set the default value of reliable_reserve_size to zero which will
disable reliable watermark check by default.

With this patch, code related to this mechanism is removed since no one use
this watermark check.

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>