[chores] Removed duplicate "verbose" formatter in settings.py

[skip ci]

Author: nemesifier

templates/openwisp2/settings.py

@@ -313,7 +313,6 @@
         'main_log': {
             'level': 'INFO',
             'class': 'logging.handlers.RotatingFileHandler',
-            'formatter': 'verbose',
             'filename': os.path.join(BASE_DIR, 'log/openwisp2.log'),
             'maxBytes': 15728640,
             'backupCount': 3,