docs(tracing): Delete inaccurate comment

`sampled` being set to `False` is not the only reason why the `_span_recorder` might be `None`. Another explanation is that the transaction was not started via `start_transaction`.

sentry_sdk/tracing.py

@@ -747,7 +747,6 @@ def finish(self, hub=None, end_timestamp=None):
             # We have no active client and therefore nowhere to send this transaction.
             return None
 
-        # This is a de facto proxy for checking if sampled = False
         if self._span_recorder is None:
             logger.debug("Discarding transaction because sampled = False")