Decapitate bot3 so it only has a drive base

Now that it is the third robot, and has no superstructure, let's check
in the code it is running.

Change-Id: Ief89efae4d4a75d837396e2731b8be4e69f55c4a
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/y2022_bot3/control_loops/python/drivetrain.py b/y2022_bot3/control_loops/python/drivetrain.py
index 8c5a2f8..638b3f3 100644
--- a/y2022_bot3/control_loops/python/drivetrain.py
+++ b/y2022_bot3/control_loops/python/drivetrain.py
@@ -14,7 +14,7 @@
 
 kDrivetrain = drivetrain.DrivetrainParams(
     J=6.0,
-    mass=58.0,
+    mass=28.0,
     # TODO(austin): Measure radius a bit better.
     robot_radius=0.39,
     wheel_radius=2.5 * 0.0254,