Blender Git Loki

Cleanup: Remove remaining unused code for DispList "bevel_split"

This was added in rB93aeb6b318a7, last changed in rB8f0a44a5d55d,
and removed in rB51b796ff1528. I assume it was a way for curves to
have split edges at corners. As far as I can tell it only ever worked
in Blender Internal, and that has been gone for years.

Another possible route here would be restoring this functionality,
but it's generally preferrable to reduce complexity in this
area of curve code than adding it back, especially in the context
of other improvements planned related to curves in geometry nodes.

Differential Revision: https://developer.blender.org/D9966