sw64: clean up unused single step support in kernel

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PNCX



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Single step is supported by GDB on sw64, and those unused code
can be removed.

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