proxy: don't set app logger level

logging is configured globally in SlapOSApp.configure_logging and:
 - if logging to file is enabled, it is logged with maximum level
 - logging on the console depends on --quiet / --verbose command line flags

Default log level of this logger was DEBUG, which is just fine: the logger
emit all messages and the file/console handler decide to output them or not

Because we configure logger after Flask initialize its own logging, for
Flask >= 1.1.2, we remove Flask logger as described in
 https://flask.palletsprojects.com/en/1.1.x/logging/#removing-the-default-handler

slapos/proxy/__init__.py

@@ -102,7 +102,14 @@ def connectDB():
 def do_proxy(conf):
   for handler in conf.logger.handlers:
     app.logger.addHandler(handler)
-  app.logger.setLevel(logging.INFO)
+
+  try:
+    # for Flask >= 1.1.2
+    from flask.logging import default_handler
+    app.logger.removeHandler(default_handler)
+  except ImportError:
+    pass
+
   setupFlaskConfiguration(conf)
   connectDB()
   app.run(host=conf.host, port=int(conf.port), threaded=True)