PM / Domains: Drop redundant code in genpd while attaching devices

The driver core together with the PM core, nowadays deals with deferring
all probes during the device system sleep phases. Therefore genpd no longer
need to care about this situation, so let's drop the corresponding code.

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