ACPI / processor: Drop unused variable from processor_perflib.c

The count variable in acpi_processor_preregister_performance() is
only initalized as 1 for one CPU and incremented when another CPU
sharing the same dependency domain is found.  It isn't referenced
anywhere else, so drop it.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>