pm: remove dummy pm_state_set/pm_state_exit_post_ops

Dummy inline functions are useful to avoid ifdeffery in code when a
certain option is not available. pm_state_set/pm_state_exit_post_ops,
are only called from the PM subsystem, so never called if CONFIG_PM=n,
that is, never surrounded with ifdeffery.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>