PM / devfreq: Remove unnecessary separate _remove_devfreq()

The _remove_devfreq() releases the all resources of the devfreq
device. This function is only called in the devfreq_dev_release().
For that reason, the devfreq core doesn't need to leave the
_remove_devfreq() separately. This patch releases the all
resources in the devfreq_dev_release() and then removes the
_remove_devfreq().

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>