sdf fix

Author: Claudio-Chies

worlds/walls.sdf

@@ -166,18 +166,6 @@
     <model name="ground_plane">
       <static>true</static>
       <link name="link">
-        <inertial>
-          <inertia>
-            <ixx>0.16666</ixx>
-            <ixy>0</ixy>
-            <ixz>0</ixz>
-            <iyy>0.16666</iyy>
-            <iyz>0</iyz>
-            <izz>0.16666</izz>
-          </inertia>
-          <mass>10</mass>
-          <pose>0 0 0 0 0 0</pose>
-        </inertial>
         <collision name="collision">
           <geometry>
             <plane>