PM: Drop generic_subsys_pm_ops

Since the PM core is now going to execute driver callbacks directly
if the corresponding subsystem callbacks are not present,
forward-only subsystem callbacks (i.e. such that only execute the
corresponding driver callbacks) are not necessary any more.  Thus
it is possible to remove generic_subsys_pm_ops, because the only
callback in there that is not forward-only, .runtime_idle, is not
really used by the only user of generic_subsys_pm_ops, which is
vio_bus_type.

However, the generic callback routines themselves cannot be removed
from generic_ops.c, because they are used individually by a number
of subsystems.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>