PB-931 Don't add exc_text manually to logging output in JSON formatter

Remove the addition of exc_text to the message directly. Instead adding
it to the record, so that it can be formatted through the django logging
facility

logging_utilities/formatters/json_formatter.py

@@ -354,9 +354,6 @@ def format(self, record):
         if self.add_always_extra:
             self._add_extra_to_message(extra, message)
 
-        if record.exc_text:
-            message['exc_text'] = record.exc_text
-
         if record.stack_info:
             message['stack_info'] = self.formatStack(record.stack_info)