logging: Stop calling basicConfig in create_logger.

From the docs:

> This function does nothing if the root logger already has handlers
> configured for it.

Which we do if we've started up Django and configured settings, and in
particular allowed Django to process `settings.LOGGING`.

So, cut it out -- all it can do is confuse people about how logging
works.

If we ever actually used the `log_format` parameter, this would be
doubly confused, because only the first call would have any effect.

zerver/lib/logging_util.py

@@ -197,7 +197,6 @@ def create_logger(name: str,
     * "log_file" should be declared in zproject/settings.py in ZULIP_PATHS.
 
     """
-    logging.basicConfig(format=log_format)
     logger = logging.getLogger(name)
 
     if log_file: