Documentation: power: Drop reference to interface.rst

It has been merged into sleep-states.rst.

Fixes: c21502efdaed ("Documentation: admin-guide: PM: Update sleep states documentation")
Signed-off-by: Jonathan Neuschäfer <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/power/index.rst

···
     drivers-testing
     energy-model
     freezing-of-tasks
-    interface
     opp
     pci
     pm_qos_interface