cfg: remove local as default image uploader

ref #9967

conf/defaults.ini

@@ -474,7 +474,7 @@ sampler_param = 1
 #################################### External Image Storage ##############
 [external_image_storage]
 # You can choose between (s3, webdav, gcs, azure_blob, local)
-provider = local
+provider =
 
 [external_image_storage.s3]
 bucket_url =
@@ -501,4 +501,4 @@ account_key =
 container_name =
 
 [external_image_storage.local]
-# does not require any configuration
+# does not require any configuration