Documentation: power: Drop reference to interface.rst

It has been merged into sleep-states.rst.

Fixes: c21502ef ("Documentation: admin-guide: PM: Update sleep states documentation")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>