docs(conf): remove internal options since its not working atm

conf/defaults.ini

@@ -404,7 +404,7 @@ url = https://grafana.net
 
 #################################### External image storage ##########################
 [external_image_storage]
-# You can choose between (s3, webdav or internal)
+# You can choose between (s3, webdav)
 provider = s3
 
 [external_image_storage.s3]