TriggerMenu: Remove unused topoStartFrom from the ChainProp class

This MR removes the unused topoStartFrom variable from the ChainProp dataclass. This was a remnant from the run 2 R21 TriggerMenu infrastructure, and @tamartin agreed it could be removed.

I'm also updating the comments above all HLT Menus to remove the mention of this property (and remove the R21-style menu entry example as a list of properties, which is now invalid as well).