Remove -v2 from all system note classes

What does this MR do and why?

Follow up to Rewrite system note CSS (!162639 - merged) and Use new CSS file for work items system notes (!163663 - merged)

• Remove unused mixin
• Remove -v2 from all system notes
• Remove old system note styles

Screenshots or screen recordings

No visual changes

Links

• ✂️ UX Paper Cuts 17.4 → Comments & system notes (#469616 - closed)