$collapsebones

From Valve Developer Community

Removes all bones that aren't connected to vertices, attachment points, ik rules, or don't animate. Vertices or attachment points that are connected to a collapsed bone are reconnected to the bones parent.

Tip: When building an animation only .mdl or a vertex-only .mdl, you'll want to do this before you create the $definebone, then remove this command.

Syntax

$collapsebones