Remove stale touch_stress docs from sensor plugin README

plugin/sensor/README.md

@@ -10,7 +10,6 @@ plugins](https://mujoco.readthedocs.io/en/latest/programming/extension.html#engi
   - [Illustration combining resolution, fields-of-view and foveal deformation](#illustration-combining-resolution-fields-of-view-and-foveal-deformation)
 
 - [Touch Stress](#touch-stress)
-  - [Example model with analytical SDF](#example-model-with-analytical-sdf)
 
 ## [Touch Grid](touch_grid.h)
 
@@ -88,63 +87,9 @@ See [touch_grid.xml](../../model/plugin/sensor/touch_grid.xml) to play with the
 
 [![touch grid illustration](https://img.youtube.com/vi/YScjmR8LwQI/0.jpg)](https://www.youtube.com/watch?v=YScjmR8LwQI)
 
-## [Touch Stress](touch_stress.h)
+## Touch Stress
 
-This sensor is based on similar concepts and parametrization as the `touch_grid`,
-while overcoming some of its limitations. In particular, the `touch_grid` can
-only provide sparse information, depending on the number of contact points
-generated. The `touch_stress` sensor can instead generate a high-resolution
-touch image. In order to do this, it requires a signed distance function (SDF)
-of the object that is in contact with the sensor. This is handled internally for
-primitives or it must be declared explicitly in the model using SDF plugins.
-
-There is one important difference with respect to the `touch_grid`: in this case,
-the force is computed in the local taxel frame and not in the frame of the sensor.
-This allows for a more intuitive interpretation of normal and tangential stresses,
-as shown in the images below.
-
-Note that in this case, the absolute values of the stresses reported by the
-sensor are unrelated to the contact forces. They are purely based on geometric
-and kinematic considerations, i.e. the SDF for the normal stress and the sliding
-velocity for the tangential contributions.
-
-### Example model with analytical SDF
-
-```xml
-<extension>
-  <plugin plugin="mujoco.sdf.gear">
-    <instance name="gear">
-      <config key="alpha" value="0"/>
-    </instance>
-  </plugin>
-  <plugin plugin="mujoco.sensor.touch_stress">
-    <instance name="touch_stress">
-      <config key="size" value="37 37"/>
-      <config key="fov" value="45 45"/>
-      <config key="gamma" value="0"/>
-      <config key="nchannel" value="3"/>
-    </instance>
-  </plugin>
-  ...
-  <asset>
-    <mesh name="gear">
-      <plugin instance="gear"/>
-    </mesh>
-  </asset>
-  ...
-  <worldbody>
-    <geom type="sdf" name="gear" mesh="gear">
-      <plugin instance="gear"/>
-    </geom>
-  ...
-  <sensor>
-    <plugin instance="touch_stress" objtype="site" objname="touch_site"/>
-  </sensor>
-</extension>
-```
-The images below show a static sphere over a gear described by an analytic SDF
-and the same sphere dragged along the x and y axes.
-
-<img src="images/normal.png" style="width: 300px;"/>
-<img src="images/tangential1.png" style="width: 300px;"/>
-<img src="images/tangential2.png" style="width: 300px;"/>
+The `touch_stress` sensor plugin was removed in MuJoCo 3.3.5 when its
+functionality was moved into the engine as the native
+[`tactile`](https://mujoco.readthedocs.io/en/latest/XMLreference.html#sensor-tactile)
+sensor. See [tactile.xml](../../model/tactile/tactile.xml) for an example model.