PM: domains: Drop the unused pm_genpd_opp_to_performance_state()

Since commit 7c41cdcd ("OPP: Simplify the over-designed pstate <->
level dance"), there is no longer any users of the
pm_genpd_opp_to_performance_state() API. Let's therefore drop it and its
corresponding ->opp_to_performance_state() callback, which also no longer
has any users.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20231127151931.47055-1-ulf.hansson@linaro.org