Remove vertcollide attribute from flex contact.

The `vertcollide` attribute in flex and flexcomp contact specifications has been removed since the next mjWarp release will support flex collisions. This attribute was previously used to enable vertex-based collisions by adding sphere geoms at flex vertices. The functionality associated with this attribute has been deprecated and removed from the XML parser, documentation, and internal data structures. Affected example XML files have also been updated.

PiperOrigin-RevId: 885595503
Change-Id: Ifbfc359cf8ba174ce10e8425b32ed309a0f0a37e

Author: quagla

dm_control/mjcf/schema.xml

@@ -1242,7 +1242,6 @@
                 <attribute name="gap" type="float"/>
                 <attribute name="internal" type="string" valid_values="true false"/>
                 <attribute name="selfcollide" type="string" valid_values="none narrow bvh sap auto"/>
-                <attribute name="vertcollide" type="string" valid_values="true false"/>
                 <attribute name="activelayers" type="int"/>
                 <attribute name="passive" type="string" valid_values="true false"/>
               </attributes>
@@ -1621,7 +1620,6 @@
                     <attribute name="gap" type="float"/>
                     <attribute name="internal" type="string" valid_values="true false"/>
                     <attribute name="selfcollide" type="string" valid_values="none narrow bvh sap auto"/>
-                    <attribute name="vertcollide" type="string" valid_values="true false"/>
                     <attribute name="activelayers" type="int"/>
                     <attribute name="passive" type="string" valid_values="true false"/>
                   </attributes>
@@ -1765,7 +1763,6 @@
                 <attribute name="gap" type="float"/>
                 <attribute name="internal" type="int"/>
                 <attribute name="selfcollide" type="string" valid_values="none narrow bvh sap auto"/>
-                <attribute name="vertcollide" type="string" valid_values="true false"/>
                 <attribute name="activelayers" type="int"/>
                 <attribute name="passive" type="string" valid_values="true false"/>
               </attributes>