sw64: remove discontiguous memory support

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



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Since sparse memory support works fine so far, discontiguous
memory support can be removed to avoid confusion.

Signed-off-by: He Sheng <hesheng@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>