Add `BusGroup#unregister(EventListener)` for single listeners#77

Jonathing wants to merge 1 commit intoMinecraftForge:masterMinecraftForge/EventBus:masterfrom

Jonathing:feat/jonathing/7.0/busgroup-unregisterJonathing/EventBus:feat/jonathing/7.0/busgroup-unregisterCopy head branch name to clipboard