mm/sharepool: Delete single-group mode

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EORS


CVE: NA

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The single-group mode has no application scenario. Therefore, the
related branch is deleted.

The boot option "enable_sp_multi_group_mode" does not take effect.

Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
Reviewed-by: Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>