PM / EM: remove em_register_perf_domain

Remove old function em_register_perf_domain which is no longer needed.
There is em_dev_register_perf_domain that covers old use cases and new as
well.

Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Quentin Perret <qperret@google.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>