perf: xgene: Remove set but not used variable 'config'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/perf/xgene_pmu.c: In function 'xgene_perf_stop':
drivers/perf/xgene_pmu.c:1055:6: warning:
 variable 'config' set but not used [-Wunused-but-set-variable]

It never used since introduction.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>