mm/sharepool: use delete_spg_node to replace some repetitive code

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I650K6



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delete_spg_node can be use to replace the code snippets as follows:
list_del(&node->proc_node);
spg->proc_num--;

Signed-off-by: Chen Jun <chenjun102@huawei.com>