powernow: don't read never initialized structure member

powernow: don't read never initialized structure member

c/s 20361:51b031b0737e removed the writing of struct
processor_performance's shared_cpu_map member, but the powernow driver
still has code to read it (though presumably that code path can't be
taken on actual hardware supported by the powernow driver). Remove the
use of the field along with the field itself.

Signed-off-by: Jan Beulich <jbeulich@suse.com>