Simplify event building logic

This will also fix some bugs where the event type being accessed
would not be available, since it is now built on-demand, without
the need to keep track for the count of each event type.

telethon/client/telegrambaseclient.py

@@ -274,11 +274,6 @@ class TelegramBaseClient(abc.ABC):
         self._event_resolve_lock = asyncio.Lock()
         self._conversations = {}
 
-        # Keep track of how many event builders there are for
-        # each type {type: count}. If there's at least one then
-        # the event will be built, and the same event be reused.
-        self._event_builders_count = collections.defaultdict(int)
-
         # Default parse mode
         self._parse_mode = markdown