remove some stale server config info

example.config.json

@@ -1,5 +1,3 @@
 {
-  "url:": "https://mx.example.com",
+  "caldav_url:": "https://mx.example.com"
-  "username": "admin@example.com",
-  "password": "password"
 }

main/config.go

@@ -7,9 +7,7 @@ import (
 )
 
 type Config struct {
-	URL      string `json:"url"`
+	URL string `json:"caldav_url"`
-	Username string `json:"username"`
-	Password string `json:"password"`
 }
 
 var serverConfig Config