Remove simple log handler at startup (#2000)

The simple logger handler is not removed if we set the `log.to=file` in
emqx.conf.

This might be an issue of OTP logger:
https://bugs.erlang.org/browse/ERL-788

I set the error_logger to silent as a workaround.

priv/emqx.schema

@@ -426,12 +426,18 @@ end}.
   {datatype, file}
 ]}.
 
-{mapping, "sasl", "sasl.sasl_error_logger", [
+{mapping, "log.sasl", "sasl.sasl_error_logger", [
   {default, off},
   {datatype, flag},
   hidden
 ]}.
 
+{mapping, "log.error_logger", "kernel.error_logger", [
+  {default, silent},
+  {datatype, {enum, [silent]}},
+  hidden
+]}.
+
 {translation, "emqx.primary_log_level", fun(Conf) ->
     cuttlefish:conf_get("log.level", Conf)
 end}.