Rework event handler to be just a series of static helper functions

- Clear out unused functions
- All functions are now static

This is because we pretty much always used it in the form:
```
EventHandler eventHandler(room, event);
eventHandler.function();
```
This simplifies it all to a single call.

src/chatbarcache.cpp

@@ -119,8 +119,7 @@ QString ChatBarCache::relationMessage() const
     }
 
     if (auto event = room->findInTimeline(m_relationId); event != room->historyEdge()) {
-        EventHandler eventhandler(room, &**event);
-        return eventhandler.getMarkdownBody();
+        return EventHandler::markdownBody(&**event);
     }
     return {};
 }