Stop <dfn>ing the "{absolute,relative}-orientation" virtual sensor types

This adapts to w3c/deviceorientation#124 -- the Device Orientation and
Orientation Sensor specifications are supposed to share the same virtual
sensor types since they provide similar data (although with different units
of measurement).

Since the Device Orientation API has broader adoption, the definition of
these sensor types have been moved there so that implementations do not need
to depend on this spec to implement WebDriver integration.

index.bs

@@ -291,7 +291,7 @@ The AbsoluteOrientationSensor Model {#absoluteorientationsensor-model}
 ----------------------------------------------------------------------
 
 The <dfn id="absolute-orientation-sensor-type">Absolute Orientation Sensor</dfn> [=sensor type=] represents the sensor described in [[MOTION-SENSORS#absolute-orientation]]. Its associated [=extension sensor interface=] is {{AbsoluteOrientationSensor}}, a subclass of {{OrientationSensor}}. Its associated [=virtual sensor type=] is
-"<code><dfn data-lt="absolute-orientation virtual sensor type">absolute-orientation</dfn></code>".
+"<code><a data-lt="absolute-orientation virtual sensor type">absolute-orientation</a></code>".
 
 For the absolute orientation sensor the value of [=latest reading=]["quaternion"] represents
 the rotation of a device's [=local coordinate system=] in relation to the <dfn export>Earth's reference
@@ -314,7 +314,7 @@ The RelativeOrientationSensor Model {#relativeorientationsensor-model}
 ----------------------------------------------------------------------
 
 The <dfn id="relative-orientation-sensor-type">Relative Orientation Sensor</dfn> [=sensor type=] represents the sensor described in [[MOTION-SENSORS#relative-orientation]]. Its associated [=extension sensor interface=] is {{RelativeOrientationSensor}}, a subclass of {{OrientationSensor}}. Its associated [=virtual sensor type=] is
-"<code><dfn data-lt="relative-orientation virtual sensor type">relative-orientation</dfn></code>".
+"<code><a data-lt="relative-orientation virtual sensor type">relative-orientation</a></code>".
 
 For the relative orientation sensor the value of [=latest reading=]["quaternion"] represents the
 rotation of a device's [=local coordinate system=] in relation to a [=stationary reference coordinate