ARM: vexpress/spc: remove unused variable perf_stat_reg

The variable 'perf_stat_reg' in ve_spc_set_performance is assigned a
value but that is never used.

So let's remove the variable 'perf_stat_reg'

Reported-by: David Binderman <dcb314@hotmail.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>