cpufreq: intel_pstate: remove cpudata::prev_cummulative_iowait

Commit 09c448d3 ("cpufreq: intel_pstate: Use IOWAIT flag in Atom
algorithm") removed the last user of cpudata::prev_cummulative_iowait.
Remove the member too.

Found by https://github.com/jirislaby/clang-struct

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Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>