sw64: remove other struct switch_stack things

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5GF7A



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Based on the previous series of patches, the do_switch_stack,
undo_switch_stack and struct switch_stack become unused. Herein
they are cleaned up.

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

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