sw64: perf: remove redundant function declarations

Sunway inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I56X48



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The inconsistent function definition and call order make previous
patch declare sw64_pmu_start() and sw64_pmu_stop() again at the
beginning. Since the order of function definition has changed, and
this part can be removed.

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