OPP: Remove unused declarations in header file

The definition of _update_set_required_opps() has been removed since
commit e37440e7 ("OPP: Call dev_pm_opp_set_opp() for required
OPPs").

Besides, the definition of _put_opp_list_kref() has been removed since
commit 03758d60 ("opp: Replace list_kref with a local counter").
Let's remove the empty declarations in header file.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>