Remove the Shell plugin and integrate it into the engine.

PiperOrigin-RevId: 760688502
Change-Id: Ia70988d42b7edf571d7cb4a4f48f8fc50b51667d

model/flex/flag.xml

@@ -0,0 +1,45 @@
+<!-- Copyright 2021 DeepMind Technologies Limited
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<mujoco model="Flag">
+<!-- Degree of Freedom (nv):  340
+     Tendons (ntendon):       288
+     Actuators (nu):          0
+     Constraints (nefc):      0
+     This model of a flag passively flapping in the wind has no constraints and is designed to
+     exert smooth dynamics computations.
+-->
+  <statistic center=".4 0 .8" extent="1.3"/>
+
+  <option wind="5 5 0" density="10" solver="CG" tolerance="1e-6">
+  </option>
+
+  <worldbody>
+    <geom name="floor" type="plane" size="0 0 .1"/>
+    <light diffuse=".6 .6 .6" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>
+    <body name="pin" pos="0 0 1.5">
+      <flexcomp type="grid" count="9 19 1" spacing=".05 .05 .05" mass="10"
+                name="flag" radius="0.001">
+        <edge equality="true" damping="0.001"/>
+        <elasticity poisson="0" thickness="1e-2" young="3e6" elastic2d="none"/>
+      </flexcomp>
+    </body>
+  </worldbody>
+
+  <equality>
+    <connect body1="flag_0" anchor="0 0 0"/>
+  </equality>
+</mujoco>
+