OPP: Remove custom OPP helper support

The only user of the custom helper is migrated to use
dev_pm_opp_set_config_regulators() interface. Remove the now unused
custom OPP helper support.

This cleans up _set_opp() and leaves a single code path to be used by
all users.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>