PM: domains: Drop redundant code for genpd always-on governor

Due to recent changes, the always-on governor is always used with a genpd
that has the GENPD_FLAG_RPM_ALWAYS_ON flag being set. This means genpd,
doesn't invoke the governor's ->power_down_ok() callback, which makes the
code in the governor redundant, so let's drop it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>