perf/x86/amd: Remove unused variable 'hwc'

'hwc' is never used in amd_pmu_enable_all(), so remove it.

Signed-off-by: Zucheng Zheng <zhengzucheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220421111031.174698-1-zhengzucheng@huawei.com