fs_storage: remove default protocol

Since protocol is under server_environment control, it seems its default value takes priority over a value set in an XML record.

Since it does really make much sense to create a storage backend without specifying
the protocol, we remove the default value.

fs_storage/models/fs_storage.py

@@ -83,7 +83,6 @@ def __init__(self, env, ids=(), prefetch_ids=()):
     protocol = fields.Selection(
         selection="_get_protocols",
         required=True,
-        default="odoofs",
         help="The protocol used to access the content of filesystem.\n"
         "This list is the one supported by the fsspec library (see "
         "https://filesystem-spec.readthedocs.io/en/latest). A filesystem protocol"